Remove stale TODOs (#4463)

## Description

Removes TODO comments that describe deliberate, final behavior as
unfinished work. No logic changes.

- `android/.../react/RNGestureHandlerRootView.kt` - drops the
`resettable lateinit` TODO from the 2021 Kotlin conversion; `rootHelper`
is assigned at most once and never reset, so the nullable var is the
intended shape
- `src/web/handlers/NativeViewGestureHandler.ts` - replaces the
activation TODO with a note that `shouldActivateOnStart` is documented
as Android-only and on web is honored internally only for the Button
role, other roles activate on real interaction on purpose
- `android/.../react/events/RNGestureHandlerStateChangeEvent.kt` - drops
two `coalescing` TODOs; state transitions are discrete and each one must
reach JS, so they are intentionally never coalesced
- `apple/RNGestureHandlerEvents.mm` - drops the same coalescing TODO
from `RNGestureHandlerStateChange` for consistency with Android

## Test plan

Comment-only changes, CI passing is sufficient.