feat: add SIRUN_READY_FD ready signal to exclude startup time

[rochdev]

Summary

• Add SIRUN_READY_FD env var: Sirun always provides a writable pipe fd to every benchmarked process
• Write any byte to it when startup is complete; Sirun resets wall.time, user.time, system.time, cpu.pct.wall.time, and (Linux) instructions to measure only the post-ready period
• CPU times are read from the live process at signal time (/proc/{pid}/stat on Linux, proc_pidinfo on macOS) and subtracted from the final RUSAGE_CHILDREN delta
• max.res.size always covers the full process lifetime (kernel-tracked peak, cannot be sampled mid-run)
• If the process exits without writing to the fd, Sirun falls back silently to full-lifetime timing

Usage

// Node.js — after all imports and startup work
require('fs').writeSync(parseInt(process.env.SIRUN_READY_FD), 'x');

# Python
import os
os.write(int(os.environ['SIRUN_READY_FD']), b'x')

# Shell
echo x >&$SIRUN_READY_FD

Test Plan

• cargo test passes (all tests including ready_signal_resets_wall_time and ready_signal_cpu_pct_bounded)
• Verify wall.time is near-zero after 500ms startup sleep + ready signal (examples/ready-signal.json)
• Verify cpu.pct.wall.time <= 100% after CPU-intensive startup + ready signal (examples/ready-signal-cpu.json)
• Verify existing benchmarks without SIRUN_READY_FD usage are unaffected

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