v0.4.0 — Hybrid data-binding, UXML codegen

Added

• Hybrid data-binding convention — Unity 6 runtime data binding
  (DataBinding/INotifyBindablePropertyChanged/[CreateProperty]) is now allowed, as a
  View-internal implementation detail behind the existing IView interfaces, for
  data-heavy screens. The MVP core is untouched; commands, clicks and navigation stay on
  ScreenSubscriptions; every binding is BindingMode.ToTarget with a nameof path
  (stringly UXML <Bindings> discouraged). Convention, walkthrough, testing story and a
  per-screen decision table: Documentation~/HYBRID-DATA-BINDING.md.
• BindableViewModel (Runtime/ViewModel/, namespace Cuvara.UIToolkit.ViewModel).
  The notifying base a binding source must derive from: Set<T>(ref field, value) guards
  with EqualityComparer<T>.Default, raises propertyChanged with the
  [CallerMemberName] property name only on real change, and returns whether anything
  changed. Notifying is mandatory because a non-notifying DataBinding source is
  version-polled by the binding system on every UI update — per-frame work the package's
  "update on data change, not per frame" contract forbids. Stays plain C#: testable with
  NUnit alone, no panel.
• EcsHud sample retrofitted as the reference hybrid screen. The imperative
  Render(caption, fraction) path is gone: the sink writes properties on a
  [CreateProperty]-annotated VitalsHudViewModel, and VitalsView.Bind assigns
  Root.dataSource and wires the label and bar once via SetBinding (the
  fraction→StyleLength conversion is a converter on the binding, so UI Toolkit types
  never leak above the View). The adapter → ViewModel → View layering and the ECS rule
  are unchanged — nothing in Runtime/Ecs/ moved. The sample's UXML (renamed
  VitalsView.uxml so the generated class matches the view) is now enrolled in the
  UXML codegen: Generated/VitalsView.uxml.g.cs is the other half of the partial View
  and is drift-checked by CI alongside the test fixture.
• Tests: Tests/Runtime/ViewModel/BindableViewModelTests.cs (plain C# — raise with
  correct name via [CallerMemberName], silence on equal values including null→null,
  same-reference and equal-but-distinct strings, return-value semantics, value and
  reference types; verified under plain dotnet as well) and
  BindableViewModelBindingTests.cs ([UnityTest] on a live UIDocument — a Set()
  reaches a bound Label and a converter-driven style.width through the real binding
  system, with no Render call).

Changed

• UXML-CODEGEN.md documents a batchmode limitation found while verifying 0.3.0 on
  a real Editor (6000.3.9f1): a -batchmode -quit session that starts with compile
  errors exits before the asset import step (exit code still 0), so the auto-regen
  postprocessor never runs in it — after an element rename breaks consuming code,
  batchmode cannot regenerate its way out. The doc lists the recovery paths (git
  checkout of the .g.cs, the menu item in an interactive Editor, or
  Tools~/UxmlCodegenCli). Docs only; no code change.