ci: WebUI installed artifact fails when the GB10 host does not become quiet #2822

Problem

The job WebUI installed artifact runs on the self-hosted GB10 runner. Its step "Run Activity performance gate with the hidden acceptance deferred" fails with:

host did not become quiet within 600s: mean runnable tasks 18 to 22 over the last 10 samples, threshold 5

This was seen on main on 2026-11-01 in the 06:54, 11:16 and 12:17 UTC runs, and on PRs #2816 and #2819. It keeps main CI red though no code is at fault, and it hides real failures in the same workflow: on #2819 a genuine compile failure sat beside it.

Acceptance Criteria

• What holds the runnable count high on the host is identified and recorded here.
• One behavior on a busy host is chosen and implemented, with the reason recorded: queue until quiet with a longer bound, serialize with whatever else uses the host, or report skipped rather than failed.
• A busy host no longer turns the job red with no code at fault.
• The gate never passes unmeasured: a skipped run is visibly reported as skipped, not as a pass.
• The threshold of 5 is not lowered or otherwise relaxed.