[ismaeelakin]

What changed

Adds stage timings for the UpgradableRwLock investigation in #404:

• uncontended acquisition and promotion measured separately
• acquisition while an ordinary reader is held
• promotion while 1, 4, or 16 ordinary read guards remain held
• promotion while a writer is queued behind the existing reservation
• a later upgradable request queued behind a writer
• --test support for the repository benchmark smoke run

This PR adds the measurement step requested in #404; it does not change the lock implementation or establish a speedup. Per-operation clock reads add overhead. Reader-promotion timings include dropping the readers and the wake/handoff, and queued acquisition includes the writer's handoff. The benchmark manually polls futures with a non-scheduling waker, so its timings exclude executor scheduling overhead. Treat the results as diagnostics.

Verification

Passed locally on Windows 11 (Intel Core i5-6300U):

• cargo x test and benchmark runs with stable rustc 1.99.1
• cargo x lint with nightly rustc 1.101.0
• cargo x bench -- --test
• cargo x bench --bench upgradable_stages -- --rounds 1000000 --repeat 5

The local timings and their run-to-run spread are recorded in issue #404.

Refs #404