feat(enterprise): add trace processor RSS benchmark harness

[Gracker]

Summary

• Adds child-process RSS sampling for owned trace_processor_shell instances and exposes it through trace processor runtime stats.
• Adds npm run benchmark:trace-rss to record startup RSS, load peak, post-load RSS, query peak, query delta, query headroom, and required §0.4.3 matrix coverage.
• Adds §0.4.3 RSS benchmark runbook plus baseline smoke results.

Scope

This is benchmark infrastructure only. README §0.4.3 intentionally remains unchecked because this checkout has no 100MB/500MB/1GB traces for scroll/startup/ANR/memory/heapprofd/vendor. The smoke run covered only under-100MB local fixtures and reported the required matrix as incomplete.

Verification

• cd backend && npx jest --runInBand --forceExit src/services/__tests__/processRss.test.ts src/scripts/__tests__/benchmarkTraceProcessorRss.test.ts
• cd backend && npm run typecheck
• cd backend && npm run test:core
• cd backend && npm run test:scene-trace-regression
• cd backend && TP_PORT_MIN=9820 TP_PORT_MAX=9849 npm run benchmark:trace-rss -- --output test-output/trace-processor-rss-benchmark-smoke.json --markdown test-output/trace-processor-rss-benchmark-smoke.md
• npm run verify:pr

Stack

Base: #83 / feature/enterprise-multi-tenant-streaming-upload

[Gracker]

已通过 commit 67856ec (merge) + 023a84a (frontend rebuild) 合入 main，关闭此 PR。