feat: enforce data/metadata separation, numeric range queries, improved docs

- Store data opaquely in add() and update() instead of spreading object
  properties into top-level metadata. data is for semantic search (HNSW),
  metadata is for structured where-filter queries (MetadataIndex).
- Fix numeric range queries in MetadataIndex — use numeric-aware comparison
  instead of lexicographic string comparison for normalized values.
- Add data field to RelateParams and Relation types for relationship content.
- Add where.type → where.noun alias in metadata-only find() path.
- Rewrite README: focused ~350 lines from 791, quick start first, feature
  showcase with mini-snippets, organized doc links, no version callouts.
- Add DATA_MODEL.md and QUERY_OPERATORS.md reference docs.
- Remove 10 outdated/redundant doc files consolidated into API reference.
- Improve JSDoc on Entity, Relation, AddParams, FindParams, and core methods.
- Fix tests asserting data properties appear in metadata (data model violation).
- Deprecate verb.source/target in favor of from/to (public) and sourceId/targetId (storage).

docs/DEVELOPER_LEARNING_PATH.md

@@ -1220,7 +1220,7 @@ const brain = new Brainy({ verbose: true })
 
 #### Resources
 
-- 📚 [API Reference](../API_REFERENCE.md) - Complete API documentation
+- 📚 [API Reference](../api/README.md) - Complete API documentation
 - 📁 [VFS Guide](../vfs/VFS_API_GUIDE.md) - Virtual Filesystem deep dive
 - 🤖 [Neural API](../guides/neural-api.md) - Advanced neural operations
 - 🌐 [Distributed Guide](../guides/distributed-system.md) - Planet-scale architecture