6zy/§11.8: co-tune topology diversity × tournament pressure — null robust

Expose tournament_k (default 2) on search()/search_staged(), threaded into
both _tournament call sites and the staged path's internal search() calls;
HOMEMAKER_TOURNAMENT_K env knob in the scaled/staged harnesses; run_6zy_ab.sh
joint niche×k grid (RESUME-able).

Result (negative, acceptable): no (niche,k) cell beats the legacy (off,k=2)
baseline. Blank-slate programme-house (5 seeds) baseline mean 4.80 fails is the
best of the 6-cell grid; every k>2 and every niche=on cell is 6.0-7.0. Niching
bites (pop_distinct 16/16 vs 4-11) but sharper pressure does not convert it to
lower fails — §11.5 'diffuses effort' null is robust to selection pressure;
plateau stays reachability-bound (confirms §11.4/§11.5).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

DESIGN.md

@@ -1108,6 +1108,81 @@ core as `fixed_circ`; threaded through `search_staged(seed_adjacency_aware=True)
   denser, more-circulation layouts), which is the canonical-encoding /
   shape-feasibility territory of `homemaker-py-9gp`.
 
+### 11.8 Topology diversity × selection pressure, co-tuned (`homemaker-py-6zy`) — DONE (negative)
+
+Premise (loose end from §11.5): structural niching was A/B'd against the legacy
+fitness-scalar dedup with selection pressure **held fixed** at a binary tournament
+(`driver._tournament`, `k=2`). §11.5's own mechanism note named the coupling as
+the reason for its null — "Maximal diversity (16/16) with the fixed tournament
+pressure just **diffuses** effort" — i.e. diversity and pressure are coupled but
+were varied as if independent: niching widens the population, but `k` was never
+**sharpened** to convert the extra exploration back into exploitation. This issue
+isolates that coupling — sweep tournament size jointly with niching to test
+whether sharper selection turns the 16/16 structural diversity into lower fails.
+The project had already pivoted to the canonical encoding (`homemaker-py-9gp`);
+this is a falsification check so the lever is not silently lost, not an expected
+win (§11.4/§11.5 both located the plateau in **reachability**).
+
+**Implementation (knob only; default-off behaviour unchanged).** Exposed
+`tournament_k: int = 2` on `search` / `search_staged`, threaded into both
+`_tournament` call sites (crossover pair + mutation parent) and all three internal
+`search()` calls of the staged path; reuses the §11.5 `genome.signature` /
+`niche_by_signature` machinery unchanged. The experiments harness reads
+`HOMEMAKER_TOURNAMENT_K` (mirrors `NICHE`) in `run_search_scaled.py` /
+`run_staged_search.py`; `experiments/run_6zy_ab.sh` runs the joint grid (RESUME-able).
+
+- *Commands (reproduce, `URB_NO_OCCLUSION=1`, 20000 evals; blank-slate seed
+  `init.dom` to match §11.5):*
+  ```bash
+  # grid: NICHE ∈ {0,1} × HOMEMAKER_TOURNAMENT_K ∈ {2,3,4}
+  NICHE=0 HOMEMAKER_TOURNAMENT_K=2 python3 experiments/run_search_scaled.py \
+    examples/programme-house 20000 <seed> examples/programme-house/init.dom scratch/out.dom
+  # harbor (staged): run_staged_search.py, seed examples/harbor-house/init.dom
+  bash experiments/run_6zy_ab.sh        # full grid → scratch/6zy/summary.tsv
+  ```
+
+- *Diversity (mechanism check) — confirmed biting.* `niche=on` holds the final
+  population at **16/16** distinct topologies at every `k`; `niche=off` sits at
+  **4–11/16**. The pressure knob is genuinely varied (`k`=2,3,4). So both levers
+  are live — the null below is not a machinery artefact.
+
+- *Fail count (the gate) — no cell beats the baseline.* Blank-slate
+  programme-house, total fails at budget over **5 seeds** (0–4), mean (sd):
+
+  | niche \ k |     k=2     |     k=3     |     k=4     |
+  |:---------:|:-----------:|:-----------:|:-----------:|
+  | **off**   | **4.80** (1.60) | 6.40 (2.50) | 6.00 (2.00) |
+  | **on**    | 6.20 (1.72) | 7.00 (1.41) | 6.60 (1.85) |
+
+  The legacy `(off, k=2)` cell is the **best** of the six (4.80); every
+  higher-pressure row and every `niche=on` row is equal-or-worse (6.0–7.0). All
+  differences sit within ~1 sd at 5 seeds, so the grid is a wash — but the central
+  tendency is unambiguous: sharpening `k` and adding niching both *slightly hurt*,
+  the opposite of the rescue the premise hypothesised. Harbor-house (staged, seed
+  0) reinforces it: `niche=on` is uniformly worse than `off` at every `k`
+  (k2 72→83, k3 77→82, k4 67→75); within the `niche=on` row higher `k` helps
+  monotonically (83→82→75) but never catches the `niche=off` row, and the best
+  cell overall (`off, k=4` = 67) is a single-seed wiggle within noise of the
+  `off, k=2` = 72 baseline.
+
+- *Why it fails — the coupling is real but points the wrong way.* Sharper
+  selection does **not** convert the extra structural diversity into lower fails;
+  if anything the 16/16 niched population at high `k` over-commits the
+  larger spread to a handful of basins and loses the occasional lucky low-fail
+  draw the smaller fitness-scalar population stumbles into. §11.5's "diffuses
+  effort" diagnosis survives co-tuning: the bottleneck is **reachability**
+  (operators + encoding cannot reach the low-fail basins), so reshaping
+  selection/population pressure cannot recover what the search space does not
+  expose — the same conclusion §11.4 reached from the comparator side and §11.5
+  from the diversity side.
+
+- *Verdict: §11.5 null is robust to selection pressure — reject `k>2` and niching
+  as defaults; binary tournament + fitness-scalar dedup stand.* `tournament_k` is
+  kept (default-2) as a reusable knob alongside `niche_by_signature`. With
+  §11.4/§11.5/§11.8 all negative on the outer loop, the residual is confirmed
+  structural: the principled lever is the canonical encoding + richer topology
+  operators (`homemaker-py-9gp`), not selection or population management.
+
 ## 12. Phase 7 — scaling validation & residual reduction (post-c4c)
 
 **Epic:** `homemaker-py-leu`. **Status:** opened 2026-06-19. Continuation of the

experiments/run_6zy_ab.sh

@@ -0,0 +1,81 @@
+#!/usr/bin/env bash
+# 6zy joint A/B: topology diversity (structural niching) x selection pressure
+# (tournament size k). §11.5 raised diversity to 16/16 but held pressure FIXED at
+# k=2; this sweep co-tunes them to test whether sharper selection converts the
+# extra structural diversity into lower fails rather than diffusing it.
+#
+# Grid: NICHE in {0,1} x HOMEMAKER_TOURNAMENT_K in {2,3,4}.
+#   (niche=0,k=2) is the legacy baseline; (niche=1,k=2) reproduces §11.5's niche
+#   column; the k in {3,4} rows are the new higher-pressure cells.
+# Seeds: programme-house 0..4 (>=5, widened from §11.5's thin 3 to clear seed
+#   noise); harbor-house staged seed 0.
+# Equal native-fitness budget, URB_NO_OCCLUSION=1. Results -> scratch/6zy/summary.tsv
+set -u
+cd "$(dirname "$0")/.."
+OUT=scratch/6zy
+mkdir -p "$OUT"
+SUMMARY="$OUT/summary.tsv"
+# RESUME=1 keeps an existing summary and skips cells already recorded (non-ERR),
+# so a killed sweep can finish only its missing cells. Default starts fresh.
+if [ "${RESUME:-0}" = 1 ] && [ -s "$SUMMARY" ]; then
+  echo "RESUME: keeping $(($(wc -l < "$SUMMARY") - 1)) existing rows"
+else
+  : > "$SUMMARY"
+  printf 'prog\tniche\tk\tseed\tfails\tseen\tpop_distinct\trestarts\ttag\n' >> "$SUMMARY"
+fi
+BUDGET=${BUDGET:-20000}
+PH_SEEDS=${PH_SEEDS:-"0 1 2 3 4"}
+HARBOR_SEEDS=${HARBOR_SEEDS:-"0"}
+MAXJOBS=${MAXJOBS:-2}
+# §11.5 reproduce-commands seed programme-house from init.dom (a bare undivided
+# plot) so the run is a true BLANK-SLATE topology search; c964…dom is a finished
+# design that would warm-start and floor trivially. Must match §11.5 for the
+# direct comparison the acceptance criteria require.
+PH_SEED_FILE=examples/programme-house/init.dom
+
+run_one() {
+  local prog=$1 harness=$2 niche=$3 k=$4 seed=$5 seedfile=$6
+  local tag="${prog}_n${niche}_k${k}_s${seed}"
+  local log="$OUT/${tag}.log"
+  if [ "${RESUME:-0}" = 1 ] && grep -qP "\t${tag}\$" "$SUMMARY" 2>/dev/null \
+      && ! grep -qP "\tERR\t.*\t${tag}\$" "$SUMMARY" 2>/dev/null; then
+    echo "SKIP $tag (already recorded)"
+    return
+  fi
+  URB_NO_OCCLUSION=1 NICHE=$niche HOMEMAKER_TOURNAMENT_K=$k \
+    python3 "experiments/$harness" \
+    "examples/$prog" "$BUDGET" "$seed" "$seedfile" \
+    "$OUT/${tag}.dom" > "$log" 2>&1
+  local fails seen popd restarts
+  fails=$(grep -oE 'best *: [0-9.eE+-]+ \([0-9]+ fails\)' "$log" | grep -oE '[0-9]+ fails' | grep -oE '[0-9]+' | tail -1)
+  seen=$(grep -oE 'diversity : [0-9]+ distinct topologies seen' "$log" | grep -oE '[0-9]+' | tail -1)
+  popd=$(grep -oE '[0-9]+/[0-9]+ distinct in final population' "$log" | head -1 | grep -oE '^[0-9]+')
+  restarts=$(grep -oE '[0-9]+ restarts' "$log" | grep -oE '[0-9]+' | tail -1)
+  printf '%s\t%s\t%s\t%s\t%s\t%s\t%s\t%s\t%s\n' \
+    "$prog" "$niche" "$k" "$seed" "${fails:-ERR}" "${seen:-?}" "${popd:-?}" "${restarts:-?}" "$tag" >> "$SUMMARY"
+  echo "DONE $tag -> ${fails:-ERR} fails (seen=${seen:-?}, popd=${popd:-?})"
+}
+
+JOBS=()
+for niche in 0 1; do
+  for k in 2 3 4; do
+    for seed in $PH_SEEDS; do
+      JOBS+=("programme-house|run_search_scaled.py|$niche|$k|$seed|$PH_SEED_FILE")
+    done
+    for seed in $HARBOR_SEEDS; do
+      JOBS+=("harbor-house|run_staged_search.py|$niche|$k|$seed|examples/harbor-house/init.dom")
+    done
+  done
+done
+
+echo "queued ${#JOBS[@]} jobs, budget=$BUDGET, maxjobs=$MAXJOBS"
+for job in "${JOBS[@]}"; do
+  IFS='|' read -r prog harness niche k seed seedfile <<< "$job"
+  while [ "$(jobs -rp | wc -l)" -ge "$MAXJOBS" ]; do wait -n; done
+  run_one "$prog" "$harness" "$niche" "$k" "$seed" "$seedfile" &
+done
+wait
+echo "ALL DONE"
+# keep header first, sort the rest
+{ head -1 "$SUMMARY"; tail -n +2 "$SUMMARY" | sort; } > "$SUMMARY.tmp" && mv "$SUMMARY.tmp" "$SUMMARY"
+cat "$SUMMARY"

experiments/run_search_scaled.py

@@ -71,6 +71,8 @@ def main() -> int:
     # fitness-scalar dedup (the "before"); RESTART_PATIENCE=<evals> enables soft
     # restarts (default off).
     niche = os.environ.get("NICHE", "0") == "1"
+    # 6zy: tournament size (selection pressure), default k=2 (legacy binary).
+    tournament_k = int(os.environ.get("HOMEMAKER_TOURNAMENT_K", "2"))
     rp = os.environ.get("RESTART_PATIENCE")
     restart_patience = int(rp) if rp else None
     adj = os.environ.get("ADJ", "1") == "1"  # s44 adjacency-aware seeding
@@ -82,6 +84,7 @@ def main() -> int:
     print(f"rng seed  : {rng_seed}")
     print(f"use_grade : {use_grade}")
     print(f"niche     : {niche}")
+    print(f"tourn_k   : {tournament_k}")
     print(f"restart_p : {restart_patience}")
     print(f"adj_aware : {adj}")
     print(f"prop_aware: {prop}")
@@ -101,6 +104,7 @@ def main() -> int:
         seed=rng_seed,
         log=lambda m: print(m, flush=True),
         use_grade=use_grade,
+        tournament_k=tournament_k,
         niche_by_signature=niche,
         restart_patience=restart_patience,
         seed_adjacency_aware=adj,

experiments/run_staged_search.py

@@ -53,6 +53,7 @@ def main() -> int:
 
     use_grade = os.environ.get("USE_GRADE") == "1"  # §11.4 graded objective A/B
     niche = os.environ.get("NICHE", "0") == "1"     # §11.5 structural niching A/B
+    tournament_k = int(os.environ.get("HOMEMAKER_TOURNAMENT_K", "2"))  # 6zy pressure knob
     rp = os.environ.get("RESTART_PATIENCE")
     restart_patience = int(rp) if rp else None
     adj = os.environ.get("ADJ", "1") == "1"  # s44/ld5 adjacency-aware seeding A/B
@@ -96,6 +97,7 @@ def main() -> int:
     print(f"rng seed  : {rng_seed}")
     print(f"use_grade : {use_grade}")
     print(f"niche     : {niche}")
+    print(f"tourn_k   : {tournament_k}")
     print(f"restart_p : {restart_patience}")
     print(f"adj_aware : {adj}")
     print(f"prop_aware: {prop}")
@@ -123,6 +125,7 @@ def main() -> int:
         seed=rng_seed,
         log=lambda m: print(m, flush=True),
         use_grade=use_grade,
+        tournament_k=tournament_k,
         niche_by_signature=niche,
         restart_patience=restart_patience,
         seed_adjacency_aware=adj,

src/homemaker_layout/driver.py

@@ -187,6 +187,7 @@ def search(
     base_p: float = 1.0,
     child_probe=None,
     use_grade: bool = False,
+    tournament_k: int = 2,
     niche_by_signature: bool = False,
     restart_patience: int | None = None,
     restart_elite: int = 1,
@@ -478,13 +479,14 @@ def search(
             tasks = []
             for _ in range(batch_n):
                 if len(pop) >= 2 and rng.random() < p_crossover:
-                    a, b = _tournament(pop, rng, _key), _tournament(pop, rng, _key)
+                    a, b = (_tournament(pop, rng, _key, k=tournament_k),
+                            _tournament(pop, rng, _key, k=tournament_k))
                     child_root, _, desc = operators.crossover(a.root, b.root, rng)
                     if child_probe is not None:
                         desc = f"{desc}|pf={a.n_fails},{b.n_fails}"
                     ratios = {**b.ratios, **a.ratios}  # primary parent wins
                 else:
-                    parent = _tournament(pop, rng, _key)
+                    parent = _tournament(pop, rng, _key, k=tournament_k)
                     child_root, desc = operators.mutate(parent.root, rng, types,
                                                         weights=mutation_weights,
                                                         reqs=reqs, base_p=base_p)
@@ -537,6 +539,7 @@ def search_staged(
     log=None,
     n_workers: int = 1,
     use_grade: bool = False,
+    tournament_k: int = 2,
     niche_by_signature: bool = False,
     restart_patience: int | None = None,
     restart_elite: int = 1,
@@ -591,7 +594,8 @@ def search_staged(
                       child_budget=child_budget, seed_budget=seed_budget,
                       p_crossover=p_crossover, seed=seed, types=types,
                       inner_kw=inner_kw, log=log, n_workers=n_workers,
-                      use_grade=use_grade, niche_by_signature=niche_by_signature,
+                      use_grade=use_grade, tournament_k=tournament_k,
+                      niche_by_signature=niche_by_signature,
                       restart_patience=restart_patience, restart_elite=restart_elite,
                       seed_adjacency_aware=seed_adjacency_aware,
                       seed_proportion_aware=seed_proportion_aware,
@@ -625,6 +629,7 @@ def search_staged(
             inner_kw=inner_kw, log=log, n_workers=n_workers,
             rank_bonus_fn=lambda root: graph.substrate_readiness(root, reqs, n_storeys),
             rank_bonus_weight=rank_bonus_weight,
+            tournament_k=tournament_k,
             niche_by_signature=niche_by_signature,
             restart_patience=restart_patience, restart_elite=restart_elite,
             seed_adjacency_aware=seed_adjacency_aware,
@@ -670,7 +675,8 @@ def search_staged(
         # §11.4: the graded objective targets the dense two-floor quality-fail
         # regime, which is Stage 2. Stage 1 keeps its readiness-biased key so the
         # substrate-selection semantics (§11.3) are unchanged.
-        use_grade=use_grade, niche_by_signature=niche_by_signature,
+        use_grade=use_grade, tournament_k=tournament_k,
+        niche_by_signature=niche_by_signature,
         restart_patience=restart_patience, restart_elite=restart_elite,
         enable_reassociate=enable_reassociate,
         feasibility_filter=feasibility_filter,