Complete runtime scheduler preemption

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Summary

• Enables multi-P runtime scheduling to run reliably with more than one P, including safe idle-M tracking, locked idle-P checks, and waking parked Ms when the last G completes.
• Adds actual SIGURG delivery from a runtime preemption thread to running M threads and routes interrupted Gs through async preemption trampolines.
• Makes ARM64 async preemption look like an injected call by preserving the interrupted LR and returning through the resume PC, and strengthens the amd64/arm64 trampoline register saves.
• Updates runtime scheduler tests to allow RUN_MAXPROCS overrides, use atomics under multi-P execution, validate multi-P progress, and prove SIGURG preempts a tight loop.

Fixes #399
Fixes #401

Notes

• Use libxev async/notify for scheduler wakeup #402 was rechecked in this slice: scheduler wakeups still call the existing run_poller_wakeup() path when Gs become runnable or when shutdown wakes parked Ms. This PR does not close Use libxev async/notify for scheduler wakeup #402 because it does not introduce per-M or global libxev async handles.

Validation

• zig fmt build.zig src/*.zig src/runtime/run_xev_bridge.zig
• clang-format -i src/runtime/run_scheduler.c src/runtime/run_scheduler.h src/runtime/tests/test_main.c src/runtime/tests/test_scheduler.c
• zig build
• zig build test (passes; command prints the expected negative missing() diagnostic from the test fixture)
• zig build test-runtime
• RUN_MAXPROCS=2 zig build test-runtime

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	runlang	505c482	Commit Preview URL Branch Preview URL	Apr 26 2026, 10:44 AM