feat(rescore): log a pass's CPU time and assertion expiries beside its elapsed time

[Iskrata]

What this PR does

Adds one always-on line after re-score: done:

re-score: cost cpu=150.0s elapsed=8813.2s cpuShare=2% assertionExpired=1 backgrounded=true

• cpu is process CPU time (getrusage, user + system) across the pass.
• assertionExpired counts how many times the noop.rescore execution assertion's expiry handler fired during the pass.

Why

It answers the question raised on #2280, which the log could not answer until now. re-score: done … in N ms is uptime, and uptime keeps running while the process is suspended, so a long elapsed time looks the same in both of these cases:

1. the pass was suspended and resumed across wakes;
2. the assertion expired and the pass ran on, detached, past its grant.

Neither leaves a cpu_resource_fatal, and only the first means a backgrounded pass reliably finishes. CPU time next to elapsed time separates them: a pass that spends a few percent of its elapsed time on CPU was mostly suspended, and assertionExpired says whether the grant ran out on the way.

It also covers the safety-net concern from that review. If pacing stops holding the limit on some device, cpuShare climbs towards the limit before any kill, and a kill still shows up on the next launch as the #1538 resuming a pass an earlier attempt could not finish line.

Type of change

• Diagnostics

How it was tested

• RescoreBackgroundSchedulerTests: the line format for the field shape and for an unreadable CPU clock, and process CPU time advancing under load. The suite passes (23 tests).
• doc_comment_lint.py is clean; parity_ratchet.py --base upstream/main --offline reports 0 errors.

Checklist

• No new build warnings introduced
• Follows the conventions in docs/CONTRIBUTING.md
• I did not commit generated output (Strand.xcodeproj/) or any secrets/keystores