Quickstart: reproduce and verify #3998

Deterministic (the gate)

PWHEADLESS=1 .venv/bin/python -m pytest tests/runtime/test_startup_torn_read_escalation.py tests/runtime/test_build_serialized.py -q

On main, before the fix, the entry-point tests fail with RuntimeError: Asset changed during preparation: …. After the fix they all pass.

Fresh-home concurrency (PR evidence, not a gate)

Use Ivan's reproducer from the #3998 comment. Set SPK_QA_SOURCE to the repo root.

SPK_QA_SOURCE=$PWD bash repro.sh 16   # ×5, one run under `taskset -c 0-3`
SPK_QA_SOURCE=$PWD bash repro.sh 32   # ×5

Expected: every process in the fresh HOME row exits 0, with 0 tracebacks, and no BUG CONFIRMED line.

Warm path unchanged

PWHEADLESS=1 .venv/bin/python -m pytest tests/runtime/test_build_serialized.py -k warm -q