Model binding source retargeting

[koculu]

Fixes stale r-model targets when bound via component :model props after source ref replacement.

What changed

• model directive now resolves its target ref at event time (prevents stale captured refs).
• Parser refresh now updates parseResult.refs before notifying parseResult.value observers.
• single-prop now preserves stable child ref linkage across ref-identity replacements and re-entangles safely.

Coverage

• Added/expanded regression tests for:
  • direct and component r-model after ref replacement
  • repeated replacements
  • parent-to-child and child-to-parent two-way sync
  • 3-level nested component :model forwarding