Pin Windows pending-delete removal for remove and prune (#1837) - #1839

Summary

• Pins Windows pending-delete handling for remove(cacheKey) and prune(maxAge: Duration.zero) (new cases in the "on Windows, an entry another remover is deleting" group), as #1825 only covered clear.
• clearHeldAtRemovalLock now fails after 10 s naming the missing .removal.lock instead of hanging to the suite timeout.
• Documents (comments only, no behavior change) that an ACL denial reading as notFound is walked past in _cacheLockKey and fails later with a different error type.
• Concurrency test compares resolved paths, so it passes with TMPDIR="$(mktemp -d)" on macOS.

Refs #1837 (items 1-4; the supervisor items remain open).

Production-readiness scope

• User-facing scope: None; tests and comments only.
• Supported platforms/paths: Windows (new tests), macOS/Linux/Windows VM (test path fix).
• Unsupported or intentionally unavailable paths: N/A, no runtime change.
• Out of scope / follow-ups: Remaining #1837 items (supervisor).

Completeness checklist

• Declared scope is fully implemented.
• Unsupported platform/option combinations: N/A, no behavior change.
• Docs/changelog: N/A, not user-observable.
• Regression coverage: new remove/prune pending-delete tests; mutation-checked (see below).
• Validation coverage registry: N/A, no public capability change.
• Security/privacy: no secrets or paths added to logs/errors.
• Follow-ups tracked in #1837.

PR type guidance

• Bugfix PR: N/A, test/comment-only; the underlying fix landed in #1825.

Test Plan

• dart run tool/prepare_workspace.dart
• dart format --output=none --set-exit-if-changed .
• dart analyze
• dart test -p vm -j 1 --exclude-tags local-only
• dart test -p chrome --exclude-tags local-only
• coverage-lib
• Other: check_platform_boundaries.dart

Matrix Evidence

Review Notes

• Independent review status: independent audit PASSED at 7d5b274 vs ec4bf4f (no P1/P2; lib diff comment-only; mutation killed exactly the 3 target tests). Two comment-precision P3s fixed in a comment-only follow-up at 403835b.
• Mutation proof: dropping the errno-5 branch in _cacheEntryDirectory on a throwaway branch (PR #1838) is expected to fail the new tests on Windows; result recorded in a comment below.
• CI status / head SHA: green at 403835b (32 pass, 9 skipped, 0 fail), also green at 7d5b274

High-risk regression review

• Classification: high-risk by path (backendRuntime); the lib/ change is comments only
• Implementation task: this PR's worker session
• Independent blocking QA task: fresh-agent exact-head audit PASSED at (earlier full audit at 7d5b274; delta comment-only); 0 P1, 0 unresolved threads
• Exact head / current base: / b78008c (merge-base with main tip ec4bf4f, which the audits diffed against)
• Production-branch deletion, bypass, or miswire proof: mutation control (closed #1838, run): dropping the errno-5 handling from _cacheLockKey fails exactly the clear, remove and prune "waits for the removal lock" tests on windows-runtime 1/2
• Affected-family real-model/artifact evidence: N/A, no model or parser behavior touched
• Explicit unavailable-family or other N/A evidence: readiness evaluator high_risk_readiness.dart on this head/base exits 2 (unverifiedPrerequisites, as designed locally); evidence is not committed
• Known PR-caused P1 regressions: 0
• Unresolved review threads: 0