Bind ModalWideNavigationRail (issue #5 follow-up)

[jonathanpeppers]

Completes the remaining actionable item from #5 that was deferred in #33.

#33 already shipped CircularProgressIndicator, LinearProgressIndicator,
SegmentedButton + both rows, WideNavigationRail, and
WideNavigationRailItem. This change adds ModalWideNavigationRail —
the modal-overlay variant. SecureTextField is still deferred on the
TextFieldState story (out of scope).

What's added

• Bridge. [ComposeBridge] ModalWideNavigationRail targets the
  stripped androidx/compose/material3/WideNavigationRailKt::ModalWideNavigationRail-k3FuEkE
  JVM name. The hash comes from the Dp @JvmInline value class on
  collapsedShadowElevation; once [generator] Phase 2: Kotlin @JvmInline value class projection dotnet/java-interop#1440 lands the
  bridge can be swapped for a direct binding call.
• Defaults enum. [ComposeDefaults] ModalWideNavigationRailDefault
  with state and content marked ! (the facade always supplies
  these — they are not enum members and not in All).
• Facade. New ModalWideNavigationRail class (ComposableContainer
  with optional Header slot). Uses the bound (non-stripped)
  WideNavigationRailStateKt.RememberWideNavigationRailState for the
  state holder, so no extra bridge is needed there.
• Sample. A "Modal rail" button on the Misc tab demonstrates the
  visibility-toggle pattern with an internal "Close" item.

Documented limitations

WideNavigationRailStates expand / collapse / snapTo methods are
Kotlin suspend functions. We don't have a LaunchedEffect /
coroutine story for C# yet, so:

1. The facade always remembers state with
   WideNavigationRailValue.Expanded, so the rail opens immediately
   when mounted.
2. Scrim-tap dismissal hides the rail visually (because
   hideOnCollapse = true) but cannot notify C#. Callers gate
   visibility on their own MutableState<bool> and add an internal
   close button — the sample shows the pattern, and the XML doc on
   ModalWideNavigationRail calls this out explicitly.

Known risk

The Misc tab still trips compose-net#42 ("LayoutNode insertAt
ArrayIndexOutOfBoundsException") when switching tabs after interacting
with several Misc widgets. ModalWideNavigationRail may share the same
underlying $changed = 0 / lambda-identity story; tracked separately
in that issue.

Verified

• dotnet test src/ComposeNet.SourceGenerators.Tests — 32/32 pass.
• dotnet build src/ComposeNet.Compose — 0 errors.
• dotnet build src/ComposeNet.Sample — 0 errors, 0 warnings.

Closes the modal-rail part of #5.

[Copilot]

Pull request overview

Adds a new facade + JNI bridge for Material3 ModalWideNavigationRail (the modal-overlay variant of WideNavigationRail) to complete the remaining actionable follow-up from issue #5, along with a sample usage in the Misc tab.

Changes:

• Introduces a new ModalWideNavigationRail composable container facade with an optional Header slot and documented limitations around dismissal/coroutines.
• Adds a [ComposeBridge] entry for the stripped WideNavigationRailKt::ModalWideNavigationRail-k3FuEkE JVM method plus a corresponding [ComposeDefaults] declaration.
• Extends the sample app’s Misc tab to demonstrate mounting/unmounting a modal rail with a “Close” item.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
src/ComposeNet.Sample/MainActivity.cs	Adds a Misc-tab demo for ModalWideNavigationRail and a visibility gate state.
src/ComposeNet.Compose/ModalWideNavigationRail.cs	New facade container that remembers rail state and calls the JNI bridge with appropriate $default masking.
src/ComposeNet.Compose/ComposeDefaults.cs	Declares ModalWideNavigationRailDefault bit positions for generator-produced defaults enum.
src/ComposeNet.Compose/ComposeBridges.cs	Adds the [ComposeBridge] stub for ModalWideNavigationRail-k3FuEkE targeting the stripped overload.