Audit layout stability signatures, measure cache, and perf invariants

[RtlZeroMemory]

Summary

• fixed layout stability signature hashing for missing layout-affecting props on supported kinds
  • stack/box side margins: mt, mr, mb, ml
  • box side border toggles: borderTop, borderRight, borderBottom, borderLeft
• expanded layout stability signature coverage with deterministic cases for:
  • layout-relevant prop inclusion
  • style-only prop exclusion
  • text/content and child-structure changes
  • determinism, unsupported-kind conservative fallback, and edge signatures
• expanded deterministic measure-cache coverage for constraint keying, axis isolation, cross-pass reuse, identity misses, edge constraints (0, Infinity, negatives), and selective-path reuse
• added deterministic perf-invariant tests for:
  • linear traversal behavior
  • signature-gated layout skip behavior
  • affected-path remeasure behavior with shared cache
  • splitPane proportional-work invariants
  • deep nesting safety
• documented Layout Stability & Caching behavior and invariants in docs/guide/layout.md

Validation

• npm run build
• npm run typecheck
• node scripts/run-tests.mjs

All commands pass on this branch.