perf(core): reactive internals refactor v1000.3.1

[nerdalytics]

Summary

• Reduces allocations in three hot paths: protectedState reader caching, stateTracking Set reuse on effect re-runs, and index-based iteration in lens path updates
• Extracts disposeEffect for full recursive cleanup of nested effects, fixing zombie refs in activeSubscribers, stateTracking, and parentSubscriber WeakMaps
• Internal simplifications: getOrCreate helper, container-to-closure refactor in derive/select/lens, unified setValueAtPath, redundant JSDoc removal
• Bumps version to 1000.3.1, adds handbook v1000.3.1 with migration docs
• Removes .github/README.md in favor of handbook
• Fixes iOS Safari safe area insets (viewport-fit=cover, env() padding, navy background on html/body)
• Bumps Node to 24.14.1

Test plan

• All 114 existing tests pass (node --test tests/**/*.ts)
• Biome check clean (npx @biomejs/biome check src/index.ts)
• TypeScript compiles (npx tsc --noEmit)
• Public API unchanged (8 function exports, 4 type exports)
• Handbook builds and serves v1000.3.1 pages correctly
• latest landing page badges reference 1000.3.1
• iOS Safari renders header below status bar with no white background bleed