perf: enable React Compiler (completes deferred Task 141 rerender audit)

[d-oit]

Summary

Task 141 (rerender audit & React 19 optimization) was deferred with the rationale that "React 19 compiler handles most automatic optimizations" — but the React Compiler is not enabled in this repo.

Analysis (2026-08-17)

• Stack: React 19.0 + Next.js 16.2.12. Next.js 16 promotes reactCompiler from experimental to stable (not enabled by default).
• No reactCompiler config in next.config.ts today.
• The codebase is already compiler-friendly:
  • Zustand selector subscriptions (useStudioStore((s) => s.entities)) throughout
  • useMemo/useCallback already applied in hot views (graph-view, mindmap-view, chat-view, library-entities)
  • No any in src/, no empty catch {} blocks, no LOC > 500 violations, no TODO/FIXME debt
  • Plan 122 already added a scoped react-hooks/incompatible-library suppression for the virtualizer hook — evidence the compiler was anticipated

Plan

1. pnpm add -D babel-plugin-react-compiler
2. Set reactCompiler: true in next.config.ts
3. Run full quality gate: lint, typecheck, unit tests, build, E2E
4. Reconcile Task 141 plan status + add a plan/ doc following repo convention

Acceptance Criteria

• pnpm run build passes (Next.js 16 + Turbopack)
• All unit tests pass
• E2E suite passes (no behavioral regressions from auto-memoization)
• No new lint/typecheck warnings
• Task 141 marked complete with verification notes

[d-oit]

Implemented in feat/react-compiler (PR to follow).

What changed

• next.config.ts: reactCompiler: true (stable in Next.js 16)
• package.json: devDependency babel-plugin-react-compiler@^1.0.0
• library-entities.tsx: 'use no memo' on useEntityListVirtualizer, EntityGrid, EntityTable

Regression found & fixed

Enabling the compiler broke the Plan 122 windowing E2E suite (library-virtualization.spec.ts, 2 tests). A/B verified: compiler off → pass, compiler on → fail (not flaky). Root cause: the virtualizer drives re-renders from DOM measurements (scroll/ResizeObserver/measureElement) that the compiler's auto-memoization doesn't track, so windowed rows stop mounting on scroll. Fixed with the documented 'use no memo' escape hatch on the virtualizer hook + consuming components.

Verification

• typecheck ✅ / lint ✅ / 2264 unit tests ✅ / build ✅
• E2E: 538 passed, 2 skipped across chromium/mobile/tablet/desktop-xl
• Compiler confirmed active in bundle (_c( markers in .next/static/chunks)
• Task 141 marked COMPLETE; plan added at plans/128-react-compiler-2026-08-17.md