ci: diff mutants only on request; the review pipeline proves them on the material before reviewing

Validate ran the three "Мутанты по диффу" shards on every push of every
branch: 48 of 56 job-hours on 08–09.09, most of them cancelled by the
next push. Mutants now run when asked — pull requests, the nightly
schedule, a push carrying a `Release:` trailer, or a dispatch with
`mutants=true` (classify-changes.mjs → `mutants_requested`); an ordinary
push runs the light checks only.

The proof moves to where it is consumed. process.yml gets a gate after
the #499 reuse step: on the code stage it looks for a dispatch Validate
run on the exact material SHA whose mutant jobs executed and passed
(scripts/validate-gate.mjs); none → it dispatches one and waits; red or
missing → the task goes back S7→S6 with the run link and the review
cycle is not spent. Spec stage and the reuse fast-path skip the gate
(`proceed=true`); all later steps branch on `proceed` in place of the
old conflict conjunct only. merge-candidate.mjs dispatches Validate on
the pushed candidate and waits for that dispatch run.

PROCESS.md/AGENTS.md: review does not start on red code; one handoff —
one push.

Mutants: mutants-run-on-every-push, review-starts-on-red-validate,
review-trusts-push-run-without-mutants, merge-waits-push-run-without-mutants.

Issue: #510
User-Visible: no

scripts/classify-changes.mjs

@@ -94,6 +94,20 @@ export function heavyGatesRequested({ eventName, headMessage, fullInput } = {})
   return hasReleaseTrailer(headMessage);
 }
 
+/**
+ * Нужны ли мутанты по диффу (#510). За 08–09.09 они съели 86 % job-минут
+ * Validate, потому что бежали на каждом промежуточном пуше и отменялись
+ * следующим. Место мутантов — кандидат: ревью-конвейер и слияние кандидата
+ * запускают Validate по кнопке с `mutants=true`, ночной прогон и PR берут
+ * полный набор, кандидат беты несёт трейлер `Release:`. Обычный push — нет.
+ */
+export function mutantsRequested({ eventName, headMessage, fullInput, mutantsInput } = {}) {
+  if (eventName === 'pull_request') return true;
+  if (eventName === 'schedule') return true;
+  if (eventName === 'workflow_dispatch') return String(fullInput) === 'true' || String(mutantsInput) === 'true';
+  return hasReleaseTrailer(headMessage);
+}
+
 /** Трейлер `Release: vX.Y.Z` в конце сообщения коммита — признак кандидата. */
 export function hasReleaseTrailer(message) {
   return /^Release:\s*v?\d+\.\d+\.\d+\S*\s*$/m.test(String(message || ''));
@@ -119,6 +133,13 @@ if (invokedDirectly) {
       fullInput: process.env.FULL_INPUT,
     });
     process.stdout.write(`heavy=${heavy ? 'true' : 'false'}\n`);
+    const mutants = mutantsRequested({
+      eventName: process.env.EVENT_NAME,
+      headMessage: process.env.HEAD_MESSAGE,
+      fullInput: process.env.FULL_INPUT,
+      mutantsInput: process.env.MUTANTS_INPUT,
+    });
+    process.stdout.write(`mutants_requested=${mutants ? 'true' : 'false'}\n`);
   } else {
     const all = process.argv.includes('--all');
     const outputs = all ? classifyAll() : classifyChanges(readFileSync(0, 'utf8'));