feat: harden plugin system wiring and add developer diagnostics

Fix critical wiring bugs that prevented plugin-provided implementations
from being used at runtime. All CRUD operations, fork/checkout/clear,
batch embedding, neural APIs, and VFS path resolution now properly
dispatch through the plugin registry.

Changes:
- Wire graphIndex to storage for getVerbsBySource() fast path
- Replace instanceof checks with duck-typing (indexIsTypeAware flag)
  so plugin HNSW indexes work in add/update/delete/search
- Add createIndex() shared helper for plugin HNSW factory
- Fix fork/checkout/clear to use plugin factories for metadataIndex,
  graphIndex, and HNSW instead of hardcoding JS constructors
- Add three-tier embedBatch priority: embedBatch > embeddings > WASM
- Skip WASM warmup/eagerEmbeddings when plugin provides embeddings
- Fix PathResolver metadataIndex access (was looking on storage)
- Use global UnifiedCache in SemanticPathResolver
- Wire plugin distance function through neural APIs
- Add diagnostics() method and CLI command for provider inspection
- Add requireProviders() for production fail-fast assertions
- Add init-time provider summary log
- Add plugin developer documentation (docs/PLUGINS.md)
- Export DiagnosticsResult type

src/plugin.ts

@@ -43,7 +43,8 @@ export interface BrainyPluginContext {
    * - 'cache'              — UnifiedCache replacement
    * - 'hnsw'               — HNSWIndex replacement
    * - 'roaring'            — RoaringBitmap32 replacement
-   * - 'embeddings'         — Embedding engine replacement
+   * - 'embeddings'         — Embedding engine replacement (single text)
+   * - 'embedBatch'         — Batch embedding engine (texts[] → vectors[])
    * - 'distance'           — Distance function overrides
    * - 'msgpack'            — Msgpack encode/decode
    *