Add runtime poll and steal benchmarks

[heartwilltell]

Summary

• add runtime work-steal and poll-delivery benchmark sources
• collect benchmark results centrally and emit stable JSON to stdout
• write the same results to bench-results.json by default, with RUN_BENCH_RESULTS available for a custom path

Fixes #412

Test Plan

• zig fmt build.zig
• clang-format -i benchmarks/runtime/bench_common.h benchmarks/runtime/bench_main.c benchmarks/runtime/bench_steal.c benchmarks/runtime/bench_poll.c
• zig build
• zig build test (passes with expected bad_check.run missing() negative diagnostic)
• zig build test-runtime
• zig build bench-runtime
• jq validation of generated bench-results.json structure

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Status	Name	Latest Commit	Preview URL	Updated (UTC)
✅ Deployment successful! View logs	runlang	42f1ce3	Commit Preview URL Branch Preview URL	Apr 26 2026, 09:30 PM