Wire runtime scheduler stress tests

[heartwilltell]

Summary

• add test_stress.c to the runtime test executable and suite
• cover high-volume spawn, producer/consumer channels, asymmetric scheduler load, tight-loop progress, concurrent nested spawn, channel ping-pong, and mixed poller I/O with CPU-bound Gs
• allow RUN_MAXPROCS overrides while keeping default runtime tests deterministic; strict multi-P/preemption assertions are opt-in with RUN_STRESS_MULTIP=1 and RUN_STRESS_PREEMPT=1

Fixes #411

Test Plan

• zig fmt build.zig
• clang-format -i src/runtime/tests/test_main.c src/runtime/tests/test_stress.c
• zig build
• zig build test (passes with expected bad_check.run missing() negative diagnostic)
• zig build test-runtime

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	runlang	61a864e	Commit Preview URL Branch Preview URL	Apr 26 2026, 09:21 PM