V3 Analysis: background analyze under a core cap and low priority

[Loufe]

Goal

Let analysis jobs run slowly in the background while the machine stays responsive for active use. A fixed core cap and below-normal scheduling replace any adaptive load monitor.

Work

• Add a background mode for analysis work with a user-set core cap.
• Apply the cap and priority through the existing containment unit: Job-object CPU rate control on Windows, nice on Linux.
• Flag throttled runs in observation provenance so their timing samples never enter unthrottled estimation cohorts.

Acceptance

• Background analysis respects the configured core cap on Windows and Linux.
• Throttled runs carry the provenance flag and are excluded from unthrottled time cohorts.
• Cancellation and shutdown settle background analysis like any other job.