feat: add an accessible dismissal path for modal sheets - #56

Refs #52.

Problem

ModalBottomSheet has no accessible dismissal path:

• On iOS the scrim is a plain UIControl that is never exposed to VoiceOver — sighted users can tap it to close the sheet, but VoiceOver users cannot reach it.
• VoiceOver's escape gesture (two-finger Z scrub) does nothing, since accessibilityPerformEscape is not implemented on the host.
• On Android the scrim is drawn directly on the host's canvas (drawScrim), so it does not exist in the accessibility tree at all, and no ACTION_DISMISS is surfaced — TalkBack has no dismiss affordance either.

Change

iOS (BottomSheetHostingView.swift)

• The scrim becomes an accessibility element with a button trait and a "Dismiss" label, but only while activating it would actually dismiss the sheet: modal, non-programmatic close detent, scrim visible — the same condition that drives scrimDismissIndex. A scrim over a programmatic-only close detent stays out of the tree, since it is decorative rather than actionable. Exposure is recomputed in updateInteractionState(), alongside the existing isUserInteractionEnabled logic.
• accessibilityPerformEscape is implemented on the hosting view. It reuses the exact scrim-tap dismissal path — factored into attemptScrimDismissal(), shared with handleScrimPress — and returns false when there is nothing to dismiss, so the gesture keeps bubbling to enclosing containers.
• The scrim is now a UIControl subclass whose accessibilityActivate sends .touchUpInside directly. VoiceOver's default activation simulates a tap at the activation point, which is unreliable when the sheet overlaps that point mid-settle.

Android (BottomSheetHostView.kt)

• The canvas-drawn scrim is exposed as a virtual accessibility node via ExploreByTouchHelper: button class, translatable "Dismiss" content description, bounds spanning from the host's top edge down to the sheet's current top, existing only under the same only-while-dismissible condition as iOS. Hover, key, and focus events are forwarded so both touch exploration and keyboard navigation reach it.
• ACTION_DISMISS is surfaced on the sheet container so TalkBack's dismiss action works while focus is inside the sheet content — the Android counterpart of accessibilityPerformEscape. Both paths funnel into attemptScrimDismissal(), which mirrors the scrim-tap path (scrimDismissIndex → snapToIndex) and emits onIndexChange, so a controlled index stays in sync.
• The virtual node's appearance/disappearance is reported via invalidateRoot() only on transitions; the check lives in updateInteractionState(), which runs on every frame of a settle.
• Adds androidx.customview:customview (for ExploreByTouchHelper).

No JS or public-API changes.

Validation

Example app on React Native 0.85.3 (New Architecture, Hermes).

Android — physical Pixel 9:

• With the "Basic modal" sheet open, the accessibility tree (uiautomator dump) contains the virtual scrim node: class="android.widget.Button", content-desc="Dismiss", clickable, focusable, bounds [0,0][1080,1689] — from the top of the screen down to the sheet's top edge.
• After the sheet closes, the node is gone from the tree.
• Touch behavior unchanged: scrim tap still dismisses; drags and snaps unchanged.

iOS — iPhone 17 simulator (iOS 26.4):

• Builds and runs; scrim-tap dismissal, drags, and snaps unchanged.
• Not yet verified with VoiceOver on a physical device (VoiceOver is not available on the simulator). I am trying to get a physical iOS device to verify the scrim exposure and the escape gesture; in the meantime, happy for anyone with device access to confirm.

Notes

• External-keyboard ESC support (raised in the issue comments) is intentionally left as a follow-up.
• The "Dismiss" label is hardcoded in English on iOS; if there is a preferred localization mechanism, I am happy to adopt it. On Android it is a string resource, so consumers can override it per locale.