Report allocation failures in fixed size PayloadBuffers (2.1.6)

[dallison]

Summary

A fixed size PayloadBuffer that runs out of space now reports the failure and keeps its existing contents. Phaser uses this to build messages in caller-provided buffers without writing past their end.

• Allocate, Realloc, SetString, AllocateMainMessage and NewMessage return nullptr. AllocateString returns an empty span. AllocateMetadata, VectorPush, VectorReserve and VectorResize return false.
• The magic word records the failure in kAllocationFailedFlag, which AllocationFailed() reports. IsValidMagic and IsMoveable ignore the flag. Readers that check the magic with kBitMapMask see the buffer as invalid.
• A small block that has no room for a new bitmap run comes from the free list, so small fixed buffers can use all of their space.
• VectorResize sets the elements it adds to zero. Before this change, previously reserved capacity and elements removed by shrinking kept stale values.

Version 2.1.6.

Testing

• bazelisk test //...: all tests pass, including new payload_buffer_test cases for exhausted buffers, the free-list fallback (SmallBlockUsesFreeListWhenRunDoesNotFit) and zeroing on resize (VectorResizeZeroesNewElements).
• Built and tested through phaser and Subspace with Bazel and CMake.