[iOS] Stop RCTIdentifierPool::dequeue from spinning forever when the pool is full

[kimseongchan0914]

Summary:

Fixes #58441.

RCTIdentifierPool::dequeue() looks for a free identifier in an unbounded while (true) loop. When every identifier is taken, the loop never exits. RCTSurfaceTouchHandler and RCTSurfacePointerHandler call it on the main thread from touchesBegan:, so the app hangs at 100% CPU until the watchdog kills it (0x8BADF00D). The issue has a symbolicated production hang report.

The pool fills up when identifiers leak over a long-lived process, for example when a touch is missing from the local registry at unregister time.

This change:

• Bounds the scan to one pass over the pool. After size steps lastIndex is back where it started, so scanning further can't find anything new.
• If no identifier is free, reclaims them all and reuses one. A reused touch identifier is a transient glitch, while the endless loop takes the whole app down.
• Initialises lastIndex, which was read as a std::bitset subscript before being assigned.

Whenever the old code terminated, the new code returns the same identifiers. This PR does not fix the identifier leak itself. It only stops a leak from turning into a hang. I'm happy to change what happens on exhaustion if you prefer a different policy.

Changelog:

[IOS] [FIXED] - Fix main thread hang in the touch handler when all touch identifiers are in use

Test Plan:

The bug reproduces against the header alone, so I tested it with standalone programs built with clang++ -std=c++20.

1. Full pool. Fill all 17 slots (the size both handlers use), then call dequeue() again:

facebook::react::RCTIdentifierPool<17> pool;
pool.reset();
for (int i = 0; i < 17; i++) pool.dequeue();
int id = pool.dequeue();

• Before: never returns (killed after 5 seconds)
• After: returns 16, and the next call returns 0

2. Same behaviour when the pool is not full. Ran the old and new headers side by side over 2,000 random dequeue/enqueue sequences of 200 operations each, never exhausting the pool, with the old lastIndex pinned to 0:

identical over 400000 operations

3. Formatting. clang-format with the repo's ObjC style reports no diff for the file.

Not run: yarn format-check-cpp, an iOS build, and an on-device check. The C++ API snapshots are unaffected because no public member changed.

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