Fix module-loader cache expiry and concurrency

[Zetazzz]

Summary

Frequent cache hits could keep module configuration cached indefinitely.
Concurrent cache misses could also start duplicate queries, and queries
that completed after invalidation could repopulate the cache with old results.

• Stop extending the TTL on cache hits.
• Share one in-flight resolution for concurrent misses with the same key
  within each loader instance.
• Prevent invalidated resolutions from repopulating the cache or removing
  newer pending work.
• Read cached values with a single get and log the cache size before clearing.

Related: constructive-io/constructive-planning#1693 (F11).

Behavior

The existing databaseId / optional apiId key, invalidate(databaseId?) API,
default 60-second TTL, and default 100-entry cache limit are retained.
TTL resolution uses the lru-cache default.

undefined results and PostgreSQL 42P01 results remain uncached.
Other resolution errors continue to propagate.

Invalidation does not cancel running queries: existing callers may still
receive their results, but those results cannot repopulate the cache.

Validation

• All 119 express-context tests passed across 8 suites.
• Full CI passed: 17/17 jobs on e71a1910b7e2.
• Tests cover cache isolation and reuse, concurrent coalescing, invalidation
  races, capacity limits, uncached absence, error propagation, and registry
  forwarding.

[tenki-reviewer]

Review complete. No issues found — approved ✅.

---

This PR introduces two new capabilities: an LRU-cached, coalescing module loader in packages/express-context and a new packages/perf-harness package for spawning benchmark workers, collecting metrics, and producing reports.

Files	Change
packages/express-context/src/loaders/* + cache-lifecycle test	Adds a bounded LRU loader with hard TTL, concurrent-miss coalescing, and exact/logical invalidation.
packages/perf-harness/src/process.ts, run.ts, index.ts, stock-worker.ts	Implements the worker spawn CLI protocol, base64url envelope, and secret handling.
packages/perf-harness/src/metrics.ts, report.ts, types.ts	Adds metrics collection and report aggregation with median/min/max and percent-change math.
packages/perf-harness/src/fixture.ts	Generates a seeded PostgreSQL fixture schema with identifier quoting and bounds checks.
packages/perf-harness/src/schedule.ts	Provides seeded deterministic benchmark case ordering.
.github/workflows/run-tests.yaml + perf-harness tests/config	Wires the new package into CI and adds unit tests across process, run, report, schedule, and fixture.

_{Reviewed commit: d5dac4d}