TypeScript: worker overlays symlink the build output, so worker builds write through to the real tree and parallel workers race — both false SURVIVED and false KILLED #31

Project shape: tests import a committed build-output directory (lib/), so the per-mutant test command is build && run tests. workers.py links everything under the root except src into each worker overlay — that includes lib/.

Consequences, observed on a TypeScript project (src/parse.ts, 126 sites):

1. Write-through: a worker's build writes through the lib/ symlink into the real tree, mutating files outside the overlay.
2. Cross-worker race: with N workers, N builds race on the same real lib/ files — a mutant's tests can execute against another worker's build.

Evidence (same mutant, both directions wrong):

• turnAt (let turn = 0 → 1): the 8-worker parallel run reported SURVIVED; running that mutant in isolation fails 4 tests (should be KILLED).
• Opposite direction also observed: mutants reported KILLED in parallel whose isolated rerun survives.

Full-run comparison: parallel fresh 75/51, serial fresh 73/53 — errors in both directions.

Related: #19 and #29 (linked directories make tests exercise the original — false SURVIVED). This adds the write-through and the parallel race, which is what produces false KILLED too, and it also means a parallel run corrupts the snapshot that #21's differential then inherits.

Suggested fix direction: exclude the build-output directory from linking (read outDir from tsconfig) and copy it into each worker instead — or give each worker a private outDir. Cloned main on 2026-11-06; happy to provide the full run logs.