chore: Route CSS transitions in common with a single platform apply hook

[MatiPl01]

Stacked on #9696 (review/merge that first). Base will retarget to main once #9696 lands.

Summary

Unifies the CSS-transition platform-routing boundary. Previously the platform exposed four hooks (canRoute, two value-blind apply paths, remove) and owned both the endpoint parsing and the reverse-shortening state. This moves all of that into the common CSSPlatformTransitionProxy:

• the proxy calls canRouteCSSProperty and parsePlatformValue directly (no hooks),
• it reverse-shortens interruptions and owns the per-view in-flight transition state,
• the platform is left with a single parsing-free hook, cssApplyTransition (animate a resolved value over a window), plus cssRemoveTransition.

So the holder's four css hooks collapse to two, and the iOS layer (REACSSPlatformTransitions) only drives Core Animation now.

Purpose

Localizes parsing, routing, and reverse-shortening in shared code, leaving the platform with pure native-animation orchestration. This is the foundation the transform-on-Core-Animation work will plug into (a transform-aware sibling of the same apply hook).

Test plan

• CSSPlatformTransitionProxy.cpp compiles on Android (real NDK flags, -Werror -Wpedantic) and iOS (host clang, iOS headers).
• The routing and reverse-shortening logic is a verbatim port of the previous Apple implementation (same reversal detection, reverseShorten, and resolved duration/start handed to Core Animation), so behavior is unchanged.
• Android holder construction is unaffected (the css fields stay trailing and value-init to null); iOS holder construction updated to the two-hook set.
• clang-format clean (pinned 19.1.7).