Harden reconciliation coverage across keyed, unkeyed, mixed, composite and deep trees

[RtlZeroMemory]

Summary

• add 5 dedicated reconciliation suites with deterministic coverage for keyed, unkeyed, mixed, composite, and deep-tree reconciliation edge cases
• add 78 new tests across those suites
• fix reconciliation gaps discovered by regression tests:
  • enforce kind-aware reuse (same slot but different kind now remounts)
  • correctly unmount unkeyed previous children when next slots are undefined
  • treat props.key generically across vnode kinds so keyed reconciliation is consistently applied
• document the reconciliation hardening matrix in docs/dev/testing.md

Why

These tests lock behavior for reorder/insert/remove, mixed keyed+unkeyed transitions, defineWidget instance/hook lifecycle, and deep nested tree mutations so reconciliation regressions are caught deterministically.

Validation

• npm run lint (fails in this environment: biome not installed)
• npm run build (fails in this environment due missing workspace type dependencies such as @types/node and @rezi-ui/testkit)
• npm run typecheck (same dependency issue as build)
• node scripts/run-tests.mjs (fails in this environment due missing workspace packages in dist test runtime)