Add CpuTime metric for per-thread CPU time measurement

[TickTockBent]

Summary

• Adds CpuTime metric implementing the Metric trait, measuring only active CPU time for the current thread (excludes sleep, I/O, lock waits)
• Unix: clock_gettime(CLOCK_THREAD_CPUTIME_ID) — nanosecond precision per-thread CPU clock
• Windows: GetThreadTimes(GetCurrentThread()) — user + kernel time for current thread
• No new dependencies — libc and windows crate features already present

This follows from the discussion in #57 where @bazhenov suggested implementing CpuTime as a separate metric after the pluggable Metric trait landed in #60:

"Now we can implement CpuTime as a separate metric to measure. I believe we have no obstacles to implementing your idea in tango code base."

Test plan

• Unit test: check_cpu_time_metric — verifies non-zero CPU time for busy work
• Integration test: cpu_time_excludes_sleep — verifies CPU time during thread::sleep is near-zero compared to a busy loop (separate binary, like rusage.rs)
• cargo test -p tango-bench — all 16 tests pass
• cargo clippy -p tango-bench — no new warnings

// ticktockbent

[bazhenov]

Thanks a lot. Will look into this shortly.

[bazhenov]

Thanks for your time and effort. I really like the implementation, it's a simple and easy to follow.

[TickTockBent]

Thanks for the review! All 4 items addressed:

1. Removed black_box from check_cpu_time_metric — the harness already black-boxes the return value, so just returning sum now.

2. Minimized Windows measurement overhead — GetCurrentThread() called once before the measurement. Only GetThreadTimes() remains in the hot path. FILETIME→nanos conversion deferred until after both samples are taken.

3. Split and relocated tests — deleted tests/cpu_time.rs, replaced with two unit tests in lib.rs:

   • cpu_time_excludes_sleep — 50ms sleep, assert < 1ms CPU time
   • cpu_time_tracks_wall_clock_during_busy_work — busy loop for 10ms wall-clock, assert CPU time within 1%

4. Stricter assertion — wall-clock bounded loop with 1% tolerance, as you suggested.

// ticktockbent

[bazhenov]

Awesome, thanks a bunch for your contribution!