feat: subtype top-level field + trackField + migrateField

Promotes `subtype?: string` to a top-level standard field on every entity,
alongside `type` / `confidence` / `weight`. Flat string, no hierarchy — the
consumer-chosen vocabulary for sub-classifying entities within a NounType
(Person → employee/customer, Document → invoice/contract, etc.).

Layer 1 — subtype field + rollup
- HNSWNounWithMetadata.subtype + STANDARD_ENTITY_FIELDS entry
- Entity / Result / AddParams / UpdateParams / FindParams threading
- add()/update() persist subtype on storageMetadata + entityForIndexing
- get()/find() route through the standard-field fast path
- subtypeCountsByType (Map<NounTypeIdx, Map<subtype, count>>) on
  BaseStorage, mirrored after nounCountsByType with the same self-heal
  rebuild and persisted to _system/subtype-statistics.json
- brain.counts.bySubtype(type, subtype?) — O(1) point + breakdown
- brain.counts.topSubtypes(type, n) — top-N by count
- brain.subtypesOf(type) — distinct subtypes seen
- find({ type, subtype }) and find({ subtype: ['a','b'] }) on the fast path

Layer 2 — trackField for other facets
- brain.trackField(name, { perType?, values? }) registers a field for
  cardinality + per-NounType breakdown stats. Backed by the aggregation
  engine (auto-defines __fieldCounts__<name>), backfill-on-define applies.
- brain.counts.byField(name, { type? }) returns value frequencies
- Optional vocabulary whitelist rejects off-vocabulary writes at add/update

Layer 3 — generic migrateField
- brain.migrateField({ from, to, readBoth?, batchSize?, onProgress? })
  streams every entity, copies the value from one path to another, and
  (unless readBoth) clears the source. Supports top-level standard fields,
  metadata.X, and data.X paths. Idempotent — safe to re-run.

Docs
- New guide: docs/guides/subtypes-and-facets.md (Layer 1 + 2 + 3)
- README, DATA_MODEL, QUERY_OPERATORS, api/README, finite-type-system,
  quick-start all treat subtype as a core primitive with anonymous example
  vocabularies (employee/customer/invoice/milestone).

Tests
- 26 new integration tests covering write/read/update/delete round-trips,
  counts rollup decrement + re-route on mutation, trackField + byField
  with and without perType, vocabulary whitelist enforcement, and
  migrateField for metadata.X → subtype and data.X → subtype paths
  including readBoth deprecation-window semantics.

Unit suite: 1468/1468 passing. Type-check + build clean.

README.md

@@ -247,6 +247,27 @@ Plugins are opt-in. Brainy never auto-imports packages unless listed in `plugins
 
 Model any domain — healthcare (`Patient → diagnoses → Condition`), finance (`Account → transfers → Transaction`), education (`Student → completes → Course`), or your own.
 
+### Subtypes — sub-classification within a NounType
+
+The 42 NounTypes are intentionally coarse. Use the top-level `subtype` field to sub-classify entities within a type — flat string, no hierarchy, your choice of vocabulary:
+
+```javascript
+await brain.add({
+  data: 'Avery Brooks — runs the AI lab',
+  type: NounType.Person,
+  subtype: 'employee'                 // 'customer', 'vendor', 'contractor', …
+})
+
+// Filter on the fast path — column-store hit, not metadata fallback:
+const employees = await brain.find({ type: NounType.Person, subtype: 'employee' })
+
+// O(1) counts via the persisted rollup:
+brain.counts.bySubtype(NounType.Person)
+// → { employee: 12, customer: 847, vendor: 34 }
+```
+
+For other facets you want counted (`status`, `source`, `role`), register them with `brain.trackField(name)`. Renaming an existing convention to `subtype`? Use `brain.migrateField({from, to})`. Full guide: **[Subtypes & Facets](docs/guides/subtypes-and-facets.md)**.
+
 **[Noun-Verb Taxonomy](docs/architecture/noun-verb-taxonomy.md)** | **[Stage 3 Canonical Reference](docs/STAGE3-CANONICAL-TAXONOMY.md)**
 
 ---