test: a mutant for the only surviving wake-up of a replaced config body (#520)

With the bodies no longer declared as Lit properties, `onBodyReplaced` is the
single path that turns a replaced config reference into an update — AC2 of
this issue rests on it, and until now only a hand-run mutation stood behind
that column. The mutant drops the notification from `setConfig`; the existing
unit in `test/config-adoption.test.mjs` reddens on it.

Verified: `node scripts/mutation-gate.mjs --id=adoption-notifies-no-host-on-config-replacement`
— «поймано 1 из 1».

Issue: #520
User-Visible: no

scripts/mutation-gate.mjs

@@ -7260,6 +7260,18 @@ const MUTANT_DEFINITIONS = [
       replace: "    _serverCfg: { state: true },\n    _devices: { state: true },",
     }],
   },
+  {
+    id: 'adoption-notifies-no-host-on-config-replacement',
+    guard: 'node --test test/config-adoption.test.mjs',
+    because: 'since #520 the bodies are no longer declared Lit properties, so this call is the '
+      + 'only thing that wakes an update when the config body is replaced: without it the '
+      + 'geometry epoch never moves and the plan keeps painting the retired body (#520 AC2)',
+    patches: [{
+      file: 'src/config-adoption.ts',
+      find: "    this.config = next;\n    this.onBodyReplaced?.('_serverCfg', previous);",
+      replace: '    this.config = next;',
+    }],
+  },
   {
     id: 'live-editor-view-mode-routes-live',
     guard: 'node --test test/live-editor.test.mjs',