Support negative margins and deterministic overlap hit testing

[RtlZeroMemory]

Summary

• allow signed int32 margins (m/mx/my/mt/mr/mb/ml) while keeping padding, pad, and gap non-negative
• add deterministic negative-margin layout fixtures and invariant checks (w/h >= 0) for overlap scenarios
• define and test deterministic overlap hit-testing rules:
  • widget tree ties resolve by stable depth-first tree order (later sibling wins)
  • layer ties resolve by zIndex, then registration order for equal zIndex
• document negative-margin behavior, constraints, overlap routing rules, examples, and gotchas in the layout guide

Why

Terminal UIs rely on overlap effects for practical composition. This update enables negative margins safely while preserving deterministic routing and non-negative size invariants.

Validation

• npm run lint
• npm run typecheck
• npm run build
• npm test
• npm run test:e2e
• npm run test:e2e:reduced
• npm run test:native:smoke