chore(post-#168): architecture-test refactor — read-once + per-name regex + structural-absence-guard docs

[jukka-matti]

Summary

Fixes the architecture-grep test flake at its algorithmic source: 16 forbidden-name checks were re-reading ~310 source files each (~5000 sync reads per run) and exceeding the 5s vitest timeout under turbo concurrent load. Now reads files once at setup and scans the in-memory cache per name.

• T2 (06d2638a) — refactor(core): read-once + alternation in architecture-grep test. beforeAll populates a CachedFile[] once; each of 16 it() blocks runs its whole-word regex against the cache. Same 16 forbidden names, same scope, per-name failure granularity preserved. Test docstring extended with "tripwire, not a wall" framing — denylist limits called out explicitly, durable answer (branded Cpk type) cross-referenced.
• T4 (1a9f7b46) — docs(testing): add architecture-test (structural-absence guard) section. New ~134-line section in docs/05-technical/implementation/testing.md documenting the pattern: when to use, when NOT, implementation (with TS code snippet matching this PR's post-refactor test + bash snippet for scripts/check-level-boundaries.sh), anti-pattern (the 3040-reads cost the original test paid), explicit limits (denylist not allowlist, substring grep, narrow scope, maintenance burden).
• T5 (14f9ca79) — docs(testing+investigations): add architecture-test cross-ref + branded-Cpk follow-up entry. (a) .claude/rules/testing.md +1 line pointing to the new docs section. (b) docs/investigations.md +60 lines new entry "Branded Cpk type as durable replacement for forbidden-name guard" — proposes opaque-type pattern mirroring ProcessHubId from PR chore(post-8f): cleanup cluster — setState fix + ProcessHubId brand + Canvas decomposition + tsc hygiene #168 (packages/core/src/processHub.ts), 4–8 task estimated scope, ADR-amendment promotion path. Final-review amend tightened forward-reference numbers (~310 files / ~5000 reads) and aligned the 3-surface cross-references.
• T6 (9c411ab9) — chore(testing): remove retry-once workaround for architecture-grep flake. In-repo: one line in testing.md (cross-ref to deleted memory replaced with credit to fix commit). Out-of-repo: deleted feedback_pr_ready_check_retry_on_grep_test.md memory + its MEMORY.md index line. Preserved the hooks/index.test.ts known-flake line in .claude/rules/testing.md (different root cause, unfixed).

Performance

State	Test cases	Total duration	Wall under turbo
Pre-refactor	16	293-367ms	hits 5s timeout intermittently
Post-refactor	16	54-58ms	623ms (verified)

~5-6× faster isolated; full turbo pnpm test completes architecture-grep test in 623ms (was hitting 5s timeout pre-fix). Adding name #17 doesn't change the cost curve (single cache pass, per-name regex on cached strings).

What this PR explicitly does NOT do

• Does not delete the architecture-grep test. It remains an ADR-073 tripwire; the branded-Cpk-type future investigation in docs/investigations.md is the durable answer, not this PR.
• Does not touch packages/hooks/src/__tests__/index.test.ts flake. Different root cause (import/env time, not file IO) — .claude/rules/testing.md retains its known-flake line for that test.
• Does not change WHAT is enforced. Same 16 forbidden names. Same package scope (@variscout/core only).

Pre-existing issue surfaced during review (NOT a PR B blocker)

apps/azure/src/features/investigation/__tests__/useCanvasViewportLifecycle.test.ts throws DatabaseClosedError: IndexedDB API missing (9 unhandled rejections in vitest teardown). Verified pre-existing on origin/main. Likely fix: import 'fake-indexeddb/auto' in apps/azure/src/setupTests.ts. Should be filed as a new docs/investigations.md entry — out of scope here.

Plan: ~/.claude/plans/how-should-we-handle-generic-sonnet.md (PR B section).

Test plan

• pnpm --filter @variscout/core exec vitest run architecture.noCrossInvestigationAggregation — 16 tests pass in 54-58ms
• Injection test: adding export const aggregateCpk = 0; triggers failure naming aggregateCpk + file path; removal restores green
• pnpm test full turbo — architecture-grep completes in 623ms
• pnpm --filter @variscout/core exec tsc --noEmit — 0 errors
• pnpm docs:check — green
• Cross-reference triangle (test docstring + docs section + investigations.md entry title) resolves consistently to "Branded Cpk type as durable replacement"
• Out-of-repo memory cleanup verified (file deleted + MEMORY.md line removed)

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