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add standalone fiber GPF repro with root cause analysis master

LLVM register allocator bug: under ReleaseSafe, the stack probe and canary instrumentation cause LLVM to skip materializing the SwitchMessage address into %rsi before fiber.zig's inline asm context switch. %rsi is left holding a stale value from Thread.current(), causing a GPF. Debug, ReleaseFast, and ReleaseSmall all pass. Only ReleaseSafe triggers it — the combination of optimization + safety instrumentation changes the code layout enough to expose the miscompilation. Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author zzstoatzz Co-author Claude Opus 4.6 (1M context) Date Commit e64d903e Parent 4748b759
+95
1 changed file