Fix fork/spawn race conditions #125
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These functions can create child processes similar to fork(), so they need to acquire the same locks as prepareForFork to prevent concurrent allocation operations from corrupting heap state. The SpawnLockGuard RAII class acquires locks in the same order as prepareForFork (heap lock, runtime lock, internal heap lock) and releases them in reverse order on destruction.
Port tests from rustforkexploration/mesh_tests to gtest: - ForkRaceWithWriters: tests mprotect race with threads actively writing during fork (verifies segfault handler correctly waits for fork) - PosixSpawn: tests posix_spawn behavior (no atfork handlers) - MixedSpawns: tests alternating posix_spawn and fork+exec - PosixSpawnWithConcurrentAllocations: stress test with concurrent allocs
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Summary
posix_spawnandposix_spawnpto acquire allocator locks before spawning, preventing races between spawn and fork operationsmprotectcalls around fork that were causing issuesFixes #123
Closes #124
Test plan