chore: Prepare platform-routed CSS transitions for more properties

[MatiPl01]

Summary

Stacked on #9552.

Prepares the platform transition machinery for routing more properties to Core Animation, with a clean platform boundary:

• PlatformValue is an abstract, platform-defined type: shared C++ only declares it (utils/platform.h); iOS defines its concrete shapes in REACSSPlatformValue.h (scalars, sizes, RGBA colors)
• parsePlatformValue is declared in shared code and implemented by the platform (iOS in REACSSValueConversion.mm); values are parsed once at routing time and routed entries carry the parsed values to the apply callback - no re-parsing anywhere
• a failed parse routes the property to the loop, including pseudo-toggle diffs (previously such values could reach the renderer and throw)
• iOS platform code split: REACSSTransitionRouting (routing predicate, owns the IOS_CSS_CORE_ANIMATION gate moved out of shared C++), REACSSValueConversion (parsing, CSS defaults, conversions, timing functions), REACSSPlatformTransitions (apply/remove only)

No behavior change: the routable property set is still opacity-only.

Test plan

• -fsyntax-only compile check on all touched translation units
• clang-format clean