mutants-nightly: weekly (Sun 03:00Z) on the X64 mutants pool, with the C324 7-day stop #5228

Operator ruling, relayed by the cop on 2026-10-12: "stop and move to weekly". This concerns mutants-nightly.yml.

Measured state, from the cop at 16:25Z: 0 green in the last 10 runs, and red or cancelled every night from 09-30 to 10-06. Under C324 item 5, a LAB check that is red or unmeasured for 7 days stops running until its owner brings it back.

Scope:

• cron goes from daily to weekly, Sunday 03:00Z. The fallback diff window goes from 24 h to 7 days.
• runs-on: [self-hosted, Linux, X64, mutants], never the ARM64 gpu runner. The perf-gx10 serialisation group is dropped with it.
• C324 7-day stop: a scheduled run that is not green, with no green run in the 7 days before it, disables the workflow. Its owner brings it back with gh workflow enable mutants-nightly.yml and a dispatch.
• scripts/check_mutants_pool.sh follows: it accepts only the mutants pool and refuses the interim ARM64 gpu docker pool.

Arming waits until the mutants runner label is live (infra#1468).

Related: #4858 (interim ARM64 pool), #4142 (nightly half of the PR cap).