fix: evaluate mount position from JS when animated reactions never fire

[icantcodefyi]

Motivation

Fixes #2721 (and the stale-closed #2690; an earlier report #2719 was auto-closed for not following the issue form): on Reanimated 4, every useAnimatedReaction attached to a sheet instance can fail to register when the instance mounts while the JS thread is busy. Derived values (detents, isLayoutCalculated) still compute, but the reaction that runs the mount position evaluation never fires — the sheet mounts parked off-screen at container height, onChange never fires, and provided animatedIndex/animatedPosition values are never written. Reproduced in production on mid-tier Android (details and Sentry field evidence in #2719).

Changes

1. Seed containerHeight at the point of measurement (BottomSheetHostingContainer): write containerHeight alongside rawContainerHeight in the same layoutState.modify, so initial layout state doesn't depend on the raw→container reaction firing. This incorporates #2689 — credit to @christian-apollo; happy to rebase if that lands first. verticalInset handling matches the reaction it replaces (modal subtracts insets, non-modal doesn't; modals pass shouldCalculateHeight={false} so in practice this path serves non-modal sheets).

2. JS-driven mount evaluation fallback (BottomSheet): a mount effect re-dispatches evaluatePosition(ANIMATION_SOURCE.MOUNT) via runOnUI every 100ms (max 20 attempts) until didAnimateOnMount flips, skipping while an animation is already RUNNING (so keyboard/mount animations are never interrupted). It runs the exact evaluation the reaction would have run — same guards, same branches — so semantics are unchanged. When reactions are healthy, the first check observes didAnimateOnMount === true and the effect retires without dispatching anything.

Both parts are needed: without (1), evaluatePosition/snapToIndex early-exit on "layout not ready" forever; without (2), the sheet still mounts invisible with perfectly correct layout state because nothing ever moves it.

Behavior when healthy

• Reactions registered fine → first interval tick sees didAnimateOnMount true → cleanup, zero UI dispatches.
• animateOnMount (mount animation running) → RUNNING guard skips until it completes, then didAnimateOnMount is true → no-op.
• Sheets mounted closed (index={-1}) → evaluation resolves the closed position exactly as the reaction path would.

Testing

Shipped as a patch-package in production (Android-heavy language-learning app): the mount-invisible reports stopped; an independent measureInWindow-based watchdog we run alongside confirms sheets now position on mount even on the devices that previously reproduced it.