Fix undefined behavior in LayoutAnimation sort comparator (#56127)

[Abbondanzo]

Summary:

shouldFirstComeBeforeSecondMutation in utils.h uses return &lhs < &rhs as a fallback when mutation types don't determine ordering. That's undefined behavior per C++ standard [expr.rel]/4, comparing pointers from different allocations (which happens during std::stable_sort's merge phase).

This wasn't a problem until the LLVM 19.x NDK upgrade changed libc++'s sort implementation. The new insertion sort phase is more aggressive about exploiting comparator correctness, and the UB now manifests as SIGSEGV in the insertion sort phase on Android.

The fix replaces the pointer comparison with a deterministic fallback using parentTag, newChildShadowView.tag, and oldChildShadowView.tag. When all properties are equal, returns false to maintain stable ordering.

Also adds MutationComparatorTest.cpp with tests covering:

• Strict weak ordering properties (irreflexivity, asymmetry, transitivity)
• Type-based ordering rules (deletes last, removes before inserts, etc.)
• Deterministic fallback by parentTag, newChildTag, oldChildTag
• Equal mutations return false (stability)
• Stress test: std::stable_sort on large mutation lists with duplicates

Changelog: [Internal]

Reviewed By: christophpurrer

Differential Revision: D95909371

[meta-codesync]

@Abbondanzo has exported this pull request. If you are a Meta employee, you can view the originating Diff in D95909371.

[meta-codesync]

This pull request has been merged in 1fdd167.

[react-native-bot]

This pull request was successfully merged by @Abbondanzo in 1fdd167

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