#620. Mutants were ~70 % of CI machine time.
1. mutation-gate.yml: a green full run (aggregator verified all six shards)
leaves a cache marker keyed by material tree + workflow SHA. A scheduled
night with the same tree and workflow, marker not older than 7 days, skips
the shards and writes "reused from run N" to the run summary. Red runs
leave no marker, so the next night runs again and still files the issue
(#472). Manual dispatch always runs the full registry. Decision is a pure
function in scripts/mutation-nightly-reuse.mjs.
2. changed_mutants: the shard plan (already computed before setup, #518) now
names the environment of its guards (plan-browser=/plan-python=, from
scripts/mutation-environment.mjs). Python + backend deps only for shards
with pytest guards, Chromium only for shards whose guards reach
Playwright; pip wheels cached. Every shard still runs and reports, so the
six mutant jobs of the review proof are unchanged.
Item 3 of the issue (smoke guards -> node --test) is out of scope here.
Issue: #620
User-Visible: no
