fix: gate memory critical on real pressure

[wbugitlab1]

Summary

• Gate memory_critical on real memory pressure: high heap ratio now also needs either critical process RSS or low host free memory.
• Keep warn/degraded memory behavior on the existing RSS floor.
• Collect host memory in health snapshots and document the new health threshold knobs.

Closes #491.

Verification

• corepack pnpm exec vitest run test/health-thresholds.test.ts test/health-monitor.test.ts --exclude test/integration.test.ts
• corepack pnpm run lint
• corepack pnpm run build
• semgrep scan --config p/default --error --metrics=off src/health/thresholds.ts src/health/monitor.ts src/types.ts test/health-thresholds.test.ts test/health-monitor.test.ts
• gitleaks protect --staged --redact

Notes

• OSV not run: no dependency, lockfile, package-manager, container, vendored, or third-party package surface changed.
• The original upstream PR wording around heapUsed / heapTotal is stale in this fork because current code already uses the effective V8/Node heap ceiling, but the missing real-pressure gate was still valid.