feat(8.0): visibility field (public/internal/system) on nouns + verbs

Adds a reserved, top-level `visibility` field (mirrors the subtype rollout):
'public' (default, surfaced) | 'internal' (developer app-internal — hidden from
default find/count/stats, opt-in via includeInternal) | 'system' (Brainy
plumbing, library-set only).

Fixes a real leak: the VFS root entity counted in getNounCount() and appeared in
find() (a fresh brain reported 1 entity). It is now visibility:'system' →
excluded from every user-facing surface. Developers also get a first-class
hidden-unless-asked tier (e.g. learned internals vs user-exposed data).

- Reserved (RESERVED_ENTITY_FIELDS / RESERVED_RELATION_FIELDS) — spoof-proof from metadata.
- Threaded through add/relate/update/transact; surfaced top-level on reads.
- Default exclusion in counts (baseStorage), find()/related() (hard candidate
  filter via excludeVisibility — keeps topK/limit correct), and stats;
  includeInternal/includeSystem opt-ins.
- VFS root marked 'system'.

Tests: visibility.test.ts 17/17 (fresh-brain getNounCount()===0, internal hidden
+ opt-in, verb symmetry, top-level surfacing, metadata-spoof rejection). Unit
1431 green; count-synchronization integration now passes (off-by-one fixed).

src/types/reservedFields.ts

@@ -35,6 +35,7 @@
  * |-----|----------------------|
  * | `noun` | the `type` param of `add()` / `update()` (stored under the key `noun`) |
  * | `subtype` | the `subtype` param |
+ * | `visibility` | the `visibility` param (`'public'` \| `'internal'`; `'system'` is Brainy-only) |
  * | `createdAt` | system-managed — set once at `add()` time |
  * | `updatedAt` | system-managed — set on every write |
  * | `confidence` | the `confidence` param |
@@ -52,6 +53,7 @@
 export const RESERVED_ENTITY_FIELDS = [
   'noun',
   'subtype',
+  'visibility',
   'createdAt',
   'updatedAt',
   'confidence',
@@ -78,6 +80,7 @@ export type ReservedEntityField = (typeof RESERVED_ENTITY_FIELDS)[number]
  * |-----|----------------------|
  * | `verb` | the `type` param of `relate()` / `updateRelation()` (stored under the key `verb`) |
  * | `subtype` | the `subtype` param |
+ * | `visibility` | the `visibility` param (`'public'` \| `'internal'`; `'system'` is Brainy-only) |
  * | `createdAt` | system-managed — set once at `relate()` time |
  * | `updatedAt` | system-managed — set on every write |
  * | `confidence` | the `confidence` param |
@@ -90,6 +93,7 @@ export type ReservedEntityField = (typeof RESERVED_ENTITY_FIELDS)[number]
 export const RESERVED_RELATION_FIELDS = [
   'verb',
   'subtype',
+  'visibility',
   'createdAt',
   'updatedAt',
   'confidence',