feat(native): grow arriving block slots into place while streaming - #1034

What/Why?

Fourth PR in the custom block renderers stack: a block replaced by a React child used to land at its full height in one frame, pushing the text below it down at once. While streamingAnimation is on, a root slot that gets its first height now grows from 0 over 200 ms instead.

Stack: #1026 (core) → #1027 (native hosts) → #1028 (JS API, docs, example) → this PR.

• How: the native stack lays a growing slot out at an eased share of its height (measurement keeps the full height, so Yoga and the height checks are unchanged). The shadow node places the React children at their final positions, so every slot view below a growing one is moved up by what is still missing above it, and the growing one is clipped to its share. The slot views move themselves (layer.transform + mask on iOS, translationY + clipBounds on Android), so touches and accessibility follow.
• When: only a slot id the view sees for the first time while streaming, with reduced motion off (and, on Android, following the animator duration scale). A static document's first render and a placeholder the reconciler recreates stay instant; a slot that leaves and comes back (a regenerated answer) grows in again. Turning streamingAnimation off cancels it.
• Structure, the same on both platforms: BlockSlotRevealAnimator (the clock), BlockSlotRevealController (shown ids and the walk over the segments), BlockSlotReveal (offset, clip and ease applied to a view), and a small BlockSlotRevealTarget protocol the slot views implement. The hosts only forward heights and restack on each frame.
• Behaviour change on Android: BlockSlotLayer.clipChildren = false, so slot content that overflows (shadows) draws past the slot, as it already does on iOS.

Known limits: slots inside blockquotes and admonitions move with the stack but do not grow; a growing card's bottom shadow appears when the clip lifts; no animation on macOS. A slotTransition prop ('grow' | 'fade' | 'none') to choose or turn this off is the next follow-up; the animator already returns raw progress for it.

Testing

• New BlockSlotRevealTests.mm (iOS) and BlockSlotRevealTest.kt (Android): measurement keeps the full height while the stack lays out the growing share; the controller walk (a growing slot moves the slot below it, an unplaced view is reset, cancel restores full heights); a slot that left grows in again; on iOS, a mask the slot did not add is left alone.
• yarn test:ios (27) and yarn test:android pass; ktlint and clang-format clean.
• Device check on the iPhone 17 simulator and a Pixel 9 emulator: the streamed Kyoto Journal demo, and the whole journal mounted at once with streaming on, so every card grows together. Text and cards stay in order with no overlap; recordings reviewed frame by frame.

PR Checklist

• Code compiles and runs on iOS
• Code compiles and runs on Android
• Updated documentation/README if applicable
• Ran example app to verify changes
• E2E tests are passing
• Required E2E tests have been added (if applicable)

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