Add runtime platform thread primitives

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Summary

• introduce run_platform.h for runtime TLS, threads, mutexes, condvars, CPU count, and Windows timer-queue callbacks
• move scheduler M/P queues and channel locks off direct pthread types
• add a Windows cooperative preemption timer path and make runtime CPU detection use the platform wrapper

Fixes #406

Test Plan

• clang-format -i src/runtime/run_platform.h src/runtime/run_scheduler.h src/runtime/run_scheduler.c src/runtime/run_chan.c src/runtime/run_runtime_api.c
• zig build
• zig build test (passes with expected bad_check.run missing() negative diagnostic)
• zig build test-runtime
• zig build -Dtarget=x86_64-windows-gnu (expected failure in libxev Windows handle support, tracked by Enable libxev Windows backend (IOCP) #407)
• zig build -Dtarget=x86_64-windows-gnu -Dlegacy-poller=true (runtime library compiles; build still fails in POSIX-only runtime test/bench helpers)

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	runlang	7e2918b	Commit Preview URL Branch Preview URL	Apr 26 2026, 09:42 PM