Add Liquid Glass button options

[Apdelrahman1911]

What

iOS 26 Liquid Glass for the button family as an iOS-only appearance knob layered on the existing variants:

NativeButton(
    text = "Continue",
    onClick = { … },
    variant = NativeButtonVariant.Primary,
    ios = NativeButtonIosOptions(background = NativeButtonIosBackground.ProminentGlass),
)

• Glass — clear adaptive capsule (real UIGlassEffect, runtime-instantiated since the class postdates the K/N bindings); content uses the system label color.
• ProminentGlass — glass tinted with the variant's container color.
• Automatic (default) — today's flat rendering, byte-identical.

Behavior contract

• iOS 26+: real glass, overlay-composited above the canvas so the effect refracts the Compose content beneath; interactive press morph from the system (the kit press dimmer steps aside).
• iOS < 26: silent fallback to Automatic (class-lookup gated, no @available).
• Android: documented no-op — the variant renders through Material as always (Robolectric-guarded).
• NativeSplitButton accepts the option but renders Automatic this release (a two-segment control needs one shared glass body — documented in buttons.md).

Demos & docs

• Sample app: Settings → Developer → "Liquid glass buttons" (capsules refracting auto-scrolling stripes; device-verified on an iPhone 17) and a showcase section under Buttons.
• docs/components/buttons.md (fallback matrix, HIG guidance), docs/interop-notes.md, docs/design-system-rules.md.

Gates

apiCheck/apiDump (additive API), kit + app unit tests incl. the new Android no-op guard, iOS compile, assembleDebug, xcodebuild.