Complete runtime metrics tracing

[heartwilltell]

Summary

• extend run_metrics_t with queue, live-G, and poll-waiter snapshot gauges
• derive gauge values from existing scheduler queues and live_g_count state
• enrich RUN_TRACE=1 JSON events with queue, poller, G, and P context

Fixes #410

Test Plan

• clang-format -i src/runtime/run_scheduler.c src/runtime/run_scheduler.h src/runtime/tests/test_scheduler.c
• zig build
• zig build test (passes with expected bad_check.run missing() negative diagnostic)
• zig build test-runtime
• RUN_TRACE=1 zig build test-runtime

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	runlang	70c1525	Commit Preview URL Branch Preview URL	Apr 26 2026, 09:13 PM