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/utils/paramValidation.ts

@@ -549,6 +549,7 @@ export function validateUpdateParams(params: UpdateParams): void {
     !params.type &&
     !params.vector &&
     params.subtype === undefined &&
+    params.visibility === undefined &&
     params.confidence === undefined &&
     params.weight === undefined
   ) {