Add WPF/XAML binding audit guidance to R3 migration skill

[runceel]

Summary

Updates the migrating-reactiveproperty-to-r3 skill to give explicit guidance on WPF/XAML data binding when migrating to R3.

Changes

• SKILL.md: Adds a new step "Audit WPF/XAML bindings" (renumbering Build/test/report to step 5). It instructs the agent to:
  • Search XAML for {Binding Xxx.Value} / ItemsSource="{Binding Xxx.Value}" after the mechanical rewrite.
  • Prefer BindableReactiveProperty<T> for UI-bound read/write properties.
  • Use ToReadOnlyBindableReactiveProperty(...) exposed as IReadOnlyBindableReactiveProperty<T> for UI-bound read-only properties.
  • Treat a successful build as insufficient — every XAML .Value binding must be backed by a bindable R3 property.
• references/rules.json: Expands the notes for the relevant rules (RP-PROP-SLIM, RP-READONLY-PROP-SLIM, RP-PROP-BINDING, RP-FROM-OBSERVABLE) with the same WPF binding guidance.

Notes

• BOM was added to both files to match the repo convention (UTF-8 with BOM).
• Docs/skill-only change; no library code affected.