Fix benchmark cell arg order and mutate_swap on undivided trees

run_urbevolve took (seed, budget, pop, cell) but cells call
fn(seed, budget, cell, **kw) — every urb-evolve cell died on TypeError,
deferred silently by pool.map. mutate_swap lacked the empty-candidates
noop guard the other operators have, crashing on init.dom-style bare
plots. Regression test: every mutation survives an undivided tree.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

experiments/benchmark_vs_urbevolve.py

@@ -50,7 +50,7 @@ def run_homemaker(seed: str, budget: int, cell: Path) -> dict:
     return {"series": series, "best_dom": out, "log_ok": proc.returncode == 0}
 
 
-def run_urbevolve(seed: str, budget: int, pop: int, cell: Path) -> dict:
+def run_urbevolve(seed: str, budget: int, cell: Path, pop: int) -> dict:
     shutil.copy(EX / "patterns.config", cell)
     shutil.copy(EX / seed, cell)
     proc = subprocess.run(

src/homemaker/operators.py

@@ -101,7 +101,10 @@ def mutate_retype(root: dom.Node, rng: np.random.Generator,
 def mutate_swap(root: dom.Node, rng: np.random.Generator,
                 types: list[str]) -> tuple[dom.Node, str]:
     child = copy.deepcopy(root)
-    li, n = _pick(rng, _owned_branches(child))
+    cands = _owned_branches(child)
+    if not cands:  # undivided topology (e.g. a bare plot seed)
+        return _finalise(child), "swap noop"
+    li, n = _pick(rng, cands)
     n.left, n.right = n.right, n.left
     return _finalise(child), f"swap {li}/{n.id or 'root'}"
 

tests/test_operators.py

@@ -83,6 +83,18 @@ def test_relink_clears_stale_below_after_base_undivide():
     assert base.by_id(cands[0].id) is not None  # parent untouched
 
 
+def test_all_mutations_survive_undivided_tree():
+    # an undivided plot (init.dom-style seed) must never crash an operator
+    bare = dom.Node(type="O", node=[[0, 0], [10, 0], [10, 8], [0, 8]],
+                    height=2.7, wall_outer=0.25, wall_inner=0.08)
+    dom._link(bare)
+    for name, op in operators.MUTATIONS.items():
+        for seed in range(3):
+            child, desc = op(bare, np.random.default_rng(seed), TYPES)
+            assert desc, name
+            canonical(child)
+
+
 def test_crossover_yields_canonical_pair():
     a = genome.decode(genome.encode(dom.load(str(CORPUS / FILES[0]))))
     b = genome.decode(genome.encode(dom.load(str(CORPUS / FILES[1]))))