Narrow gherkin mutation runner-worker to the changed scenario

The DB soft Gherkin mutation re-ran the whole feature for every mutant;
with a fresh LevelDB world per example, a 48-mutation feature exceeded
900s. The runner-worker now diffs the mutated IR against the base feature
at <work>/base/feature.json (derived from the mutation path, since
job.work_dir may be mutation-specific) and runs only the changed
scenario/example. Safe because each scenario uses an isolated world.
The same feature now completes in ~74s.

By architect.

psf-memo-db/acceptance/lib/runner-worker.js

@@ -22,11 +22,43 @@
 
 import readline from 'node:readline'
 import fs from 'node:fs'
+import path from 'node:path'
 
 console.log = (...args) => console.error('[worker]', ...args)
 
 const { runFeature } = await import('./runtime.js')
 
+// A gherkin mutation changes one example value in one scenario. Every scenario
+// runs in its own isolated LevelDB world, so the other scenarios cannot be
+// affected by the mutation. Narrow the IR to the changed scenario/example so a
+// mutant does not re-run the whole feature. Falls back to the full IR when the
+// base feature is missing or its shape differs.
+function narrowToChanged (ir, basePath) {
+  try {
+    const base = JSON.parse(fs.readFileSync(basePath, 'utf8'))
+    if (!base || !Array.isArray(base.scenarios) || base.scenarios.length !== ir.scenarios.length) return ir
+
+    for (let i = 0; i < ir.scenarios.length; i++) {
+      const scenario = ir.scenarios[i]
+      const original = base.scenarios[i]
+      if (JSON.stringify(scenario) === JSON.stringify(original)) continue
+
+      const examples = Array.isArray(scenario.examples) ? scenario.examples : []
+      const originalExamples = Array.isArray(original.examples) ? original.examples : []
+      if (examples.length === originalExamples.length) {
+        const changed = examples.filter((ex, j) => JSON.stringify(ex) !== JSON.stringify(originalExamples[j]))
+        if (changed.length > 0) {
+          return { ...ir, scenarios: [{ ...scenario, examples: changed }] }
+        }
+      }
+      return { ...ir, scenarios: [scenario] }
+    }
+    return ir
+  } catch (err) {
+    return ir
+  }
+}
+
 const rl = readline.createInterface({
   input: process.stdin,
   output: process.stdout,
@@ -49,7 +81,12 @@ rl.on('line', async (line) => {
 
   try {
     const ir = JSON.parse(fs.readFileSync(job.feature_json, 'utf8'))
-    const report = await runFeature(ir)
+    // The mutated feature lives at <work>/mutations/<id>/feature.json, so the
+    // base feature is two levels up. Do not trust job.work_dir (the mutator may
+    // pass the mutation-specific directory).
+    const basePath = path.resolve(path.dirname(job.feature_json), '..', '..', 'base', 'feature.json')
+    const narrowed = narrowToChanged(ir, basePath)
+    const report = await runFeature(narrowed)
     respond({
       id: job.id,
       outcome: report.failures === 0 ? 'test_success' : 'test_failure',