Add a reusable Graphile performance harness

[Zetazzz]

Summary

Adds a reusable Graphile performance harness that runs each measurement in a fresh Node process, isolating build timing and memory samples between cases. Suites supply serializable case definitions and a dedicated worker entry for their build and validation logic. This consolidates #1716 and incorporates subsequent review fixes.

• Seeded scheduling and repetitions; build-only timing; memory snapshots, deltas, and peak RSS.
• Schema-group/hash validation, runtime query checks, per-case validation, and a minimal upstream Graphile baseline worker.
• Explicit database URL and worker-config CLI arguments; atomic JSON reports with database URL redaction, including custom-worker errors.
• Configurable worker deadline (five minutes by default). A timed-out worker is killed and confirmed closed before continuing; unconfirmed cleanup after another five seconds stops scheduling and produces a failed partial report.
• PostgreSQL fixture preparation that closes the pool on failure and preserves primary, rollback, release, and shutdown errors.
• The baseline worker emits one terminal result after releasing its service. Measurement and release failures both fail the run, preserving both redacted diagnostics when they occur together.
• Separate library and CLI entries: CJS/ESM library imports do not start the CLI; the built cperf executable and local workspace script run the command interface.
• The private package is included in the CI unit-test matrix; its tests build the package before exercising generated entries.

Validation

• Frozen-lockfile install; CJS and ESM builds.
• 65 tests across 7 suites, covering worker timeouts, cleanup failures, library imports, and the built CLI.
• Local workspace CLI smoke test and type checks for the new tests.
• ESLint, Prettier check, and git diff --check.
• Full CI on 27f4dc7331e5: 17/17 jobs passed, including 65/65 perf-harness tests.

[tenki-reviewer]

Review complete. 🟡 1 medium

💬 Inline comments (1)

• 🟡 Success result overwritten by cleanup failure — stock-worker.ts:75

🧹 Nitpicks (2) — 🟢 2 low

• 🟢 require.main throws in the ESM entry (index.ts:13) — src/index.ts line 13 runs require.main === module, but tsconfig.esm.json emits the same file as ESM (module: esnext) into dist/esm, which package.json exposes as module: esm/index.js; in ESM scope require is undefined, so importing the module entry throws ReferenceError: require is not defined at module load.
• 🟢 cperf bin points at nonexistent root index.js (package.json:10) — packages/perf-harness/package.json line 10 declares bin.cperf as index.js and lines 6-8 set main/module/types to package-root paths, but tsconfig.json sets outDir: "dist", so the build lands in dist/index.js and dist/esm.

---

This PR introduces a new performance test harness package under packages/perf-harness: a CLI (cperf), a worker process runner, seeded scheduling, metrics aggregation, report generation, and a Postgres fixture helper, together with unit tests and a CI job. The code is well-structured and well-tested, but the review surfaced a few packaging and cleanup issues.

Files	Change
src/index.ts, src/stock-worker.ts, package.json, tsconfig*.json	Adds the harness entry points and build config; the ESM build exposes require.main and the cperf bin points at a nonexistent root path.
src/stock-worker.ts, src/process.ts, src/run.ts	Implements worker spawning and result collection; a cleanup failure can overwrite a valid success result.
src/metrics.ts, src/schedule.ts, src/report.ts, src/fixture.ts	Adds metrics, scheduling, reporting, and DB fixture logic with unit tests.
__tests__/*, .github/workflows/run-tests.yaml	Adds tests for the harness and wires it into CI.

_{Reviewed commit: 8f89a88}

[tenki-reviewer]

Adds a new packages/perf-harness package (CLI, worker process, metrics, scheduling, reporting, DB fixtures) plus its tests and CI wiring, with three packaging/cleanup defects to address.

Key findings

• 🟡 Success result overwritten by cleanup failure — stock-worker.ts:75