test(miner-concurrency): add parallel-attempt collision regression suite

[JSONbored]

This depends on the git-worktree-per-attempt allocator (sibling Phase 6 issue, feat(miner-concurrency): add git-worktree-per-attempt allocator) landing first — there is nothing to regression-test until acquire/release exist. Filing this now so the dependency is visible in the tracker, but a contributor should pick up the allocator issue first (or claim both together).

Once the allocator exists, this is a focused regression suite proving the concurrency guarantees the allocator issue's deliverables promise, rather than just its happy path: that concurrent acquire() calls never hand out the same worktree path, that release() correctly frees a path for reuse, that the configured concurrency cap is actually enforced under simultaneous requests (not just checked sequentially), and that a simulated crash (an allocation recorded as active with no live owner) is reclaimed on the next startup rather than either double-allocated or leaked forever.

Deliverables

• test/unit/miner-worktree-allocator-collisions.test.ts (or fold into the allocator's own primary test file if that ends up small enough — match whatever filename the allocator issue actually lands, since test/unit/miner-*.test.ts is the established location per test/unit/miner-claim-ledger.test.ts and siblings).
• Concurrent-acquire test: fire N simultaneous acquire() calls (e.g. via Promise.all) and assert all N returned paths are distinct.
• Cap-enforcement test: request more than the configured concurrency limit simultaneously and assert the excess requests are rejected or queued (per whatever contract the allocator issue settles on), never silently over-allocated.
• Release/reuse test: release() a path, then acquire() again, and assert the freed path becomes available (not permanently retired).
• Crash-recovery test: seed the allocator's persisted state with an "active" allocation that has no live owner, restart/reopen the store, and assert it is reclaimed rather than treated as still-live.
• Follow the existing miner-package test conventions (node:sqlite-backed local-store tests use a temp DB path per test — see test/unit/miner-claim-ledger.test.ts / test/unit/miner-run-state.test.ts for the setup/teardown pattern already in use).

References

• Sibling issue: feat(miner-concurrency): add git-worktree-per-attempt allocator (Phase 6) — hard dependency, must land first.
• test/unit/miner-claim-ledger.test.ts, test/unit/miner-run-state.test.ts — existing SQLite-backed local-store test setup/teardown pattern to reuse for the allocator's own store.

[linear-code]

JSO-3285