fix: the adoption bodies are no longer declared Lit properties (#520)

#500 gave `_serverCfg` and `_layout` prototype accessors but left them in
`static properties`. Lit marks such a property `wrapped` and, on the FIRST
update, force-writes it into `changedProperties` with an `undefined` old
value even though nobody assigned anything (`reactive-element.js:249-252`
and `:880-886`). `willUpdate` reads that as a config replacement, raises
`_cfgEpoch`, the memoized model key changes, and a 60-room house builds and
paints its model a second time: measured 19 update cycles, 4 builds and 4
epochs against 18 / 3 / 3 before #500, worth ~550 ms of `modelReadyMs` and
the same on `firstStableRenderMs` (3355 against a 3000 ceiling).

The declaration goes; the bodies stay reactive through the owner —
`_adoption` → `onBodyReplaced` → `requestUpdate(field, previous)` — which
needs no declaration: `getPropertyOptions` falls back to the default and
`changed.has('_serverCfg')` works as before. `noAccessor: true` would not
help, `wrapped` is set before that flag is read. The trap is written above
`static properties`, where someone would put the declaration back.

`cache.entries.cleanFloor` returns to 100 in both interaction budgets: the
120 entries were the extra epoch re-keying the per-room cache, not a
property of the design — the reasoning in 914e8402 was wrong.

Witness: test/config-adoption-ownership.test.mjs pins that neither body is
declared; the mutant `adoption-bodies-declared-reactive` puts the
declaration back and reddens it.

The boot diagnostics of the previous three commits touch four private
members, so they are declared in the performance contract: `_buildModel` and
`_cfgEpoch` outright (both exist in every supported comparison base), and the
adoption entry point as a current/legacy pair — #500 turned the private
`_adoptStructuralResponses` into the public `_adoptAuthoritative`, and an
undeclared rename would have the counter report zero adoptions instead of
failing.

The same commits carried a `node_modules` symlink: `.gitignore` had the
pattern with a trailing slash, which does not cover a symbolic link, and
`git add -A` in a sandbox worktree committed it. The link is removed and the
pattern loses the slash; a mutant run on this branch failed with `EEXIST` on
it.

Issue: #520
User-Visible: no

demo/performance/budgets-interaction-smoke.json

@@ -96,7 +96,7 @@
     "hardMaxGrowthBytes": 67108864
   },
   "cacheEntries": {
-    "cleanFloor": 180,
+    "cleanFloor": 100,
     "glowClip": 200,
     "wallUnion": 1,
     "openingTunnel": 1,

demo/performance/budgets-large-house-interaction.json

@@ -118,7 +118,7 @@
     "noiseAllowanceBytes": 16777216
   },
   "cacheEntries": {
-    "cleanFloor": 180,
+    "cleanFloor": 100,
     "glowClip": 200,
     "wallUnion": 1,
     "openingTunnel": 1,

demo/performance/card-contract.mjs

@@ -18,6 +18,7 @@ export const LARGE_HOUSE_CARD_CONTRACT = Object.freeze({
   methods: Object.freeze([
     '_baseVb',
     '_bindingStatus',
+    '_buildModel',
     '_cancelDecorGesture',
     '_checkSpacePhysicalGeometry',
     '_decorBoxOf',
@@ -40,6 +41,7 @@ export const LARGE_HOUSE_CARD_CONTRACT = Object.freeze({
     '_booting',
     '_bootSoft',
     '_cameraTransition',
+    '_cfgEpoch',
     ...CACHE_FIELDS,
     '_continuity',
     '_cursorPt',
@@ -72,13 +74,20 @@ export const LARGE_HOUSE_CARD_CONTRACT = Object.freeze({
   // bundles moved it into ResizeController. A comparison target must expose
   // one of the two explicit shapes; the current member retains its object
   // type check and is also verified against current production source.
+  // #500 replaced the private structural adoption with a gated public entry
+  // point. The boot diagnostics (#520) count adoptions through whichever of
+  // the two the measured bundle owns; an undeclared rename would silently
+  // report zero adoptions instead of failing.
   fieldAlternatives: Object.freeze([
     Object.freeze({ current: '_resize', legacy: '_rszDrag' }),
+    Object.freeze({ current: '_adoptAuthoritative', legacy: '_adoptStructuralResponses' }),
   ]),
   fieldTypes: Object.freeze({
+    _adoptAuthoritative: 'function',
     _booting: 'boolean',
     _bootSoft: 'boolean',
     _cameraTransition: 'object',
+    _cfgEpoch: 'number',
     _cleanFloorCache: 'map',
     _devices: 'array',
     _continuity: 'object',