fix(#1001): make bug-969 hardening tests hermetic + move build tsbuildinfo out of shipped tree (regression from #996) (#1002)

* fix(#969): make bug-969 hardening tests hermetic and move build tsbuildinfo out of shipped tree (regression from #996)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* fix(#969): self-heal legacy bin-local tsbuildinfo and make sentinel test hermetic (adversarial-review follow-ups)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* chore(changeset): set pr number to 1002

* docs(#1001): record DEFECT.SHARED-ARTIFACT-MUTATION-IN-CONCURRENT-TEST anti-pattern in CONTEXT.md

---------

Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

issue-996-regression.md

@@ -0,0 +1,26 @@
+## Summary
+
+`next` is currently **red** across the full-test matrix (macOS 22/24, Windows 22) and the ubuntu-24 coverage leg. Root-caused to commit `88e30d53` (PR #996, which closed #969 "fix stale-build flake"). The commit right before it (`d617735b`) was green.
+
+## Root cause
+
+`#996` did two things that interact badly with the **concurrent** `node --test` runner:
+
+1. Added `tests/bug-969-test-infra-flake-hardening.test.cjs` whose two destructive tests `fs.unlinkSync` the **real shared** `gsd-core/bin/lib/core.cjs` (and the real build tsbuildinfo) mid-run, rebuilding/restoring in `finally`. Because `node --test` runs files concurrently, any other test that `require`s `core.cjs` during that window fails with `Cannot find module .../gsd-core/bin/lib/core.cjs`. This is a textbook **real-race test** (forbidden by `RULESET.TESTS.delete-bad-tests`).
+2. Set `tsBuildInfoFile: "gsd-core/bin/tsconfig.build.tsbuildinfo"` — placing mutable build state **inside `gsd-core/bin/`**, a directory install tests copy recursively via `fs.cpSync`. A concurrent rebuild writing/unlinking that file races the copy → `copyfile ENOENT`.
+
+Symptom: ~40–50 tests fail per leg with `MODULE_NOT_FOUND`/`ENOENT`; leg-asymmetric (timing-sensitive). Reproduced locally on the first iteration of running the bug-969 test concurrently with `state.test.cjs` + `install.test.cjs`.
+
+## Impact
+
+`next` red ⇒ branch protection blocks **all** PRs from merging (Required tests fails).
+
+## Fix (fix-forward)
+
+- Make the two destructive `bug-969` tests **hermetic** — exercise a parameterized `ensureBuiltArtifacts(overrides)` against a throwaway temp TS project; never touch real `gsd-core/bin/lib`.
+- Move `tsconfig.build.tsbuildinfo` **out of `gsd-core/bin/`** to the repo root (gitignored); self-heal stale bin-local copies on persistent workspaces.
+
+## Verification
+
+- 10× concurrent race check clean (was iter-1 repro before).
+- Full unit suite: 4775 pass / 0 fail through the modified runner.