ci(mutants): skip the nightly registry on a proven tree; install only the shard's environment

#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

scripts/mutation-gate.mjs

@@ -23,6 +23,7 @@ import {
   LEDGER_SCHEMA, readLedger, recordCaught, splitByLedger, witnessFingerprint,
 } from './mutation-evidence.mjs';
 import { attributeSetupFailure } from './mutation-attribution.mjs';
+import { guardEnvironment, planEnvironment, planEnvironmentLines } from './mutation-environment.mjs';
 import { MUTATION_OUTCOME, isProofOutcome } from './mutation-guard-outcome.mjs';
 
 const repoRoot = fileURLToPath(new URL('..', import.meta.url));
@@ -215,6 +216,11 @@ export async function main(argv) {
   // ДО установки окружения (npm ci, python, Chromium ≈ 3 минуты на шард).
   if (argv.includes('--plan-only')) {
     console.log(`plan=${toRun.length}`);
+    // #620: и какое окружение нужно гардам плана — шаг ставит только его.
+    const exists = (file) => existsSync(join(repoRoot, file));
+    const read = (file) => (exists(file) ? readFileSync(join(repoRoot, file), 'utf8') : '');
+    const need = planEnvironment(toRun, (guard) => guardEnvironment(guard, { read, exists }));
+    for (const line of planEnvironmentLines(need)) console.log(line);
     reportPlanMetrics();
     return 0;
   }