Add acceptance runner adapter and persist mutation state

Add the persistent runner adapter for gherkin-mutator and commit the
differential/acceptance mutation manifests written by the approved tools
during the architect verification pass for the post-memo feature.

By architect.

acceptance/lib/runner-worker.js

@@ -0,0 +1,68 @@
+/*
+  Persistent runner adapter for the APS gherkin-mutator.
+
+  The mutator starts this process (once per worker) and sends mutation jobs
+  over newline-delimited JSON on stdin. Each job carries the path to a mutated
+  feature JSON IR; this worker evaluates it through the same acceptance runtime
+  and step handlers used by the normal acceptance pipeline and replies with the
+  runner outcome.
+
+  Protocol (mutator-spec.md):
+    request:  { "id", "feature_json", "generated_dir", "work_dir" }
+    response: { "id", "outcome", "output", "error", "duration" }
+      outcome: test_success | test_failure | infrastructure_error
+
+  test_failure (acceptance failed) -> mutation killed
+  test_success (acceptance passed) -> mutation survived
+*/
+
+'use strict'
+
+const readline = require('node:readline')
+const fs = require('node:fs')
+const { runFeature } = require('./runtime')
+
+const rl = readline.createInterface({
+  input: process.stdin,
+  output: process.stdout,
+  terminal: false
+})
+
+rl.on('line', async (line) => {
+  const started = Date.now()
+  const respond = (payload) => {
+    process.stdout.write(`${JSON.stringify(payload)}\n`)
+  }
+
+  let job
+  try {
+    job = JSON.parse(line)
+  } catch (err) {
+    respond({ id: 'unknown', outcome: 'infrastructure_error', output: '', error: `bad job: ${err.message}`, duration: Date.now() - started })
+    return
+  }
+
+  try {
+    const ir = JSON.parse(fs.readFileSync(job.feature_json, 'utf8'))
+    const report = await runFeature(ir)
+    respond({
+      id: job.id,
+      outcome: report.failures === 0 ? 'test_success' : 'test_failure',
+      output: report.results.map((r) => `${r.status} ${r.name}`).join('\n'),
+      error: '',
+      duration: Date.now() - started
+    })
+  } catch (err) {
+    respond({
+      id: job.id,
+      outcome: 'infrastructure_error',
+      output: '',
+      error: err.message,
+      duration: Date.now() - started
+    })
+  }
+})
+
+rl.on('close', () => {
+  process.exit(0)
+})