fix: the authoritative adoption keeps its own task (#520)

The r1 diagnosis was wrong, and the measurement in the code review proved
it: removing the two declarations from `static properties` left the cold
start at 19 update cycles, 4 model builds and 4 config epochs, exactly the
numbers of the bug. Lit's forced first-update change does mark `_serverCfg`
changed, but at that moment the body and `_cfgEpochPreservedConfig` are both
null, `preserveGeometry` is true and the epoch does not move. The comment
above `static properties` now says that; the declaration still stays out,
because two owners of one reactivity is what #500 removed.

The real cause is the `await`. Before #500 everything from
`_adoptStructuralResponses` to the end of the load ran in one task: the
adopted bodies, `_adoptInitialSpace`, the viewport restore, `_loadOk`, and
the device seeding — whose `_syncNewDevices`/`_seedHiddenDevices` write the
config back — all landed in a single Lit update. #500 made the adoption an
async sequence, so the caller resumes one microtask later, after Lit has
already painted the adopted config; the seeding writes then arrive as a
second config epoch, a second model build and a second paint of a 60-room
house.

`GatedAdoptionInput` gains `afterAdopt`, the mirror of `beforeAdopt`: it
runs synchronously at the end of the sequence, before the promise resolves.
`_loadFromServer` moves the viewport restore, `_loadOk` and the device
rebuild into it — `_syncNewDevices` refuses to write before `_loadOk`, so
the order inside the hook matters — and the load tail now rebuilds devices
only when nothing was adopted. `_reloadConfigOnly` takes the same route.

Measured with the project's own runner, 7 samples per profile, base
`a44fbd37` against this tree (Chromium 152, sandbox):

  interaction  modelReadyMs 761.3 ≤ 950.56 (base 731.2)
               firstStableRenderMs 2567.2 ≤ 3000 (base 2542.7)
               longTask.maxSingleMs 690 ≤ 921 · cache.entries.cleanFloor 100
  isometric    modelReadyMs 1252.9 ≤ 1499.76 (base 1249.8)
               firstStableRenderMs 1378 ≤ 1610.16 (base 1341.8)

Boot diagnostics on both trees: 18 update cycles, 3 model builds, 3 config
epochs, with the same epoch trace — the candidate is no longer
distinguishable from the base.

Witnesses. `config-adoption.test.mjs` queues a microtask at the start of
the adoption and pins that `afterAdopt` runs before it — the probe fails the
moment the hook crosses an await; `config-adoption-ownership.test.mjs` pins
the wiring in the card and the hook's place in the sequence. Mutants
`adoption-tail-defers-caller-hook` (defers the hook by one microtask) and
`authoritative-load-seeds-devices-after-the-await` (drops the rebuild from
the hook) redden them.

The initial View graph grows 40 B gzip, so the #438 ceiling is recentred
300 300 → 300 400 with the usual dated note; measured 299 812 B keeps 588 B
above and 1 412 B below the band. The overall 301 066 B budget and the #367
headroom debt are untouched.

Issue: #520
User-Visible: no

scripts/mutation-gate.mjs

@@ -7251,9 +7251,10 @@ const MUTANT_DEFINITIONS = [
     id: 'adoption-bodies-declared-reactive',
     guard: 'npx tsc -p tsconfig.test.json && node scripts/fix-test-build.mjs '
       + '&& node --test --test-name-pattern="#520" test/config-adoption-ownership.test.mjs',
-    because: 'declaring the adoption bodies as Lit properties marks them `wrapped`, and the '
-      + 'forced first-update change raises the config epoch: the model of a 60-room house is '
-      + 'built and painted a second time, +550 ms to the first stable frame (#520)',
+    because: 'since #500 `_adoption` owns the reactivity of these bodies; declaring them again '
+      + 'makes Lit a second owner, marks them `wrapped` and force-writes a change on the first '
+      + 'update — today harmless only because both sides are null at that moment, and a '
+      + 'spurious config epoch as soon as a warm cache arrives earlier (#520 AC1)',
     patches: [{
       file: 'src/houseplan-card.ts',
       find: "    _devices: { state: true },",
@@ -7272,6 +7273,30 @@ const MUTANT_DEFINITIONS = [
       replace: '    this.config = next;',
     }],
   },
+  {
+    id: 'adoption-tail-defers-caller-hook',
+    guard: 'node --test test/config-adoption.test.mjs',
+    because: 'the caller hook closes the adoption task; deferring it by even one microtask lets '
+      + 'Lit paint the adopted config first, and the device seeding that follows costs a second '
+      + 'config epoch, a second model build and a second paint of a 60-room house (#520 AC6)',
+    patches: [{
+      file: 'src/config-adoption.ts',
+      find: '  input.afterAdopt?.();',
+      replace: '  void Promise.resolve().then(() => input.afterAdopt?.());',
+    }],
+  },
+  {
+    id: 'authoritative-load-seeds-devices-after-the-await',
+    guard: 'node --test --test-name-pattern="#520" test/config-adoption-ownership.test.mjs',
+    because: 'the cold-start regression of #520 itself: seeding the devices outside the adoption '
+      + 'task writes the config back after Lit already painted it, 19 update cycles, 4 model '
+      + 'builds and 4 config epochs against 18/3/3 (#520 AC6)',
+    patches: [{
+      file: 'src/houseplan-card.ts',
+      find: '          this._loadOk = true;\n          rebuildDevices();',
+      replace: '          this._loadOk = true;',
+    }],
+  },
   {
     id: 'live-editor-view-mode-routes-live',
     guard: 'node --test test/live-editor.test.mjs',