All ifcopenshell calls (from_string, filter_elements, geom.iterator) now
run inside a short-lived child process via run_sandboxed(). A SIGSEGV or
SIGABRT in the child returns HTTP 422 instead of killing the service worker.
Configurable timeout (IFCURL_SANDBOX_TIMEOUT, default 120s) and optional
resource limits (IFCURL_SANDBOX_MEMORY_MB, IFCURL_SANDBOX_CPU_SECS).
The T3 GUID cache still works: the sandboxed pipeline returns resolved
GUIDs on selector miss so the parent can populate the cache; on hit the
parent passes cached GUIDs and the child converts them to step IDs.
Tests use a sync_sandbox fixture that runs run_sandboxed synchronously so
mock patches remain visible; four dedicated sandbox tests cover normal
return, exception propagation, segfault detection, and timeout.
Closes ifcurl-vm5.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
