Io.Uring fiber context-switch GPFs under ReleaseSafe on x86_64-linux. Reproduced with a 30-line minimal program (repro_evented.zig): - Debug (safety ON, opts OFF): passes - ReleaseFast (safety OFF, opts ON): passes - ReleaseSmall (safety OFF, opts ON): passes - ReleaseSafe (safety ON, opts ON): GPF in fiber.zig contextSwitch This is a zig 0.16-dev stdlib bug (confirmed in both dev.3059 and dev.3066) — the LLVM optimizer miscompiles safety-checked code in the fiber context switch path. Fix: build with ReleaseFast instead of ReleaseSafe. Backend stays on Io.Evented (fibers on io_uring), eliminating thread-per-PDS. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>