Bind M3 Expressive newcomers: SplitButton, ButtonGroup, LoadingIndicator

[jonathanpeppers]

The Material 3 Expressive update (M3 1.4+) added a handful of new shape-driven components that aren't wrapped yet. Lower priority than the core button/FAB gaps, but worth tracking together.

Missing composables

Composable	Kt class
SplitButtonLayout (+ leading/trailing button slots)	SplitButtonKt, SplitButtonDefaults
ButtonGroup	ButtonGroupKt
LoadingIndicator (containerless)	LoadingIndicatorKt
ContainedLoadingIndicator	LoadingIndicatorKt

Notes

• SplitButtonLayout is a multi-slot composable (leading button + trailing dropdown button) — model with named slot properties on the facade type, like AlertDialog.
• ButtonGroup exposes a ButtonGroupScope receiver for children with per-item weight modifiers — same scope-receiver pattern as the segmented button rows. Can probably ship without the scope helpers first.
• LoadingIndicator is the new wavy/morphing indicator that replaces CircularProgressIndicator in M3 Expressive contexts. ContainedLoadingIndicator is the "with surface background" variant.
• Verify these APIs are present in the version of Xamarin.AndroidX.Compose.Material3 we're pinned to before starting; if they're newer than 1.4.0.x we may need a NuGet bump first.

Tracking

Long-term dotnet/java-interop#1440.

[jonathanpeppers]

Verified against Xamarin.AndroidX.Compose.Material3Android 1.4.0.3 (currently pinned in Directory.Build.targets) — the M3 Expressive Composables aren''t there yet. Unpacked classes.jar from the AAR and grepped for the three Kt classes:

$ jar tf classes.jar | grep -E "(SplitButton|ButtonGroup|LoadingIndicator)"
androidx/compose/material3/tokens/ButtonGroupSmallTokens.class
androidx/compose/material3/tokens/ConnectedButtonGroupSmallTokens.class
androidx/compose/material3/tokens/LoadingIndicatorTokens.class
androidx/compose/material3/tokens/SplitButtonLargeTokens.class
androidx/compose/material3/tokens/SplitButtonMediumTokens.class
androidx/compose/material3/tokens/SplitButtonSmallTokens.class
androidx/compose/material3/tokens/SplitButtonXLargeTokens.class
androidx/compose/material3/tokens/SplitButtonXSmallTokens.class

Only the design tokens ship in 1.4.0 stable — the actual SplitButtonKt, ButtonGroupKt, LoadingIndicatorKt Composable functions land in the 1.5.0-alphaXX line (current is 1.5.0-alpha21).

NuGet only publishes 1.4.0.x for Xamarin.AndroidX.Compose.Material3Android — no 1.5.0-alpha binding exists, and binding an alpha package isn''t in scope for this repo.

Parking this issue. Will revisit once Xamarin.AndroidX.Compose.Material3Android 1.5.0 (stable) lands on NuGet, at which point we can bump the pin in Directory.Build.targets and add the three facades via the standard [ComposeBridge] + [ComposeFacade] recipe documented in .github/copilot-instructions.md.