Add growable runtime stacks

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Summary

• Enables growable green-thread stacks by default on POSIX using the reserved virtual-memory path that already existed in the runtime.
• Adds generated C stack probes at function entry and implements run_stack_check() / run_morestack() so stack use commits additional pages before overflow.
• Keeps the SIGSEGV handler as a safety net and also handles SIGBUS on macOS for reserved-page stack faults.
• Tracks stack watermarks at scheduler yield points and decommits underused pages with hysteresis so stacks shrink after deep calls unwind.
• Adds runtime tests for stack growth and shrinkage, plus a codegen assertion for emitted stack probes.

Fixes #403
Fixes #404

Implementation Note

This completes growth without copying live stack frames: the runtime reserves each G's maximum stack range up front and doubles the committed portion downward as needed. That preserves active frame pointers and saved contexts while matching the issue's growth behavior and keeping run_morestack() non-stubbed.

Validation

• zig fmt src/codegen_c.zig
• 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; command prints the expected negative missing() diagnostic from the test fixture)
• zig build test-runtime

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	runlang	ec8a379	Commit Preview URL Branch Preview URL	Apr 26 2026, 03:34 PM