chore(8.0): delete vectorStore:mmap wiring — dead in the 8.0 provider world

The mmap-vector backend was a 2.x-era accelerator for the JS HNSW read path:
a native provider registered under 'vectorStore:mmap' supplied a single-file
mmap'd vector store, and JsHnswVectorIndex tried it before per-entity storage
reads with lazy write-back migration.

In the 8.0 provider world nothing registers that key — the native plugin's
3.0 line replaces the entire vector index under the 'vector' provider key
(vectors live inside its own index file), and the open-core path never had
an mmap provider. The wiring is unreachable; per the no-unwired-code rule
it goes away:

- src/hnsw/mmapVectorBackend.ts deleted (175 LOC)
- wireMmapVectorBackend() + its init call site removed from brainy.ts
- VectorStoreMmapProvider + VectorStoreMmapInstance contracts removed
  from plugin.ts
- JsHnswVectorIndex loses the vectorBackend field, setVectorBackend(),
  and the mmap-first branches in getVectorSafe/preloadVectors — the
  storage + UnifiedCache path is now the single read path
- tests/unit/hnsw/mmap-vector-backend.test.ts deleted

The 7.x line keeps the wiring and got the capacity-NaN bugfix as 7.31.3
(see the platform handoff mmap thread). EntityIdMapperProvider stays — the
connections codec and the id mapper still implement it.

1403/1403 tests, build clean.

src/brainy.ts

@@ -40,10 +40,8 @@ import { PluginRegistry } from './plugin.js'
 import type {
   BrainyPlugin,
   BrainyPluginContext,
-  GraphCompressionProvider,
-  VectorStoreMmapProvider
+  GraphCompressionProvider
 } from './plugin.js'
-import { MmapVectorBackend } from './hnsw/mmapVectorBackend.js'
 import { ConnectionsCodec } from './hnsw/connectionsCodec.js'
 import { TransactionManager } from './transaction/TransactionManager.js'
 import { RevisionConflictError } from './transaction/RevisionConflictError.js'
@@ -604,15 +602,6 @@ export class Brainy<T = any> implements BrainyInterface<T> {
         this.graphIndex = graphIndex
       }
 
-      // Wire the mmap-vector backend (2.4.0 #2). When the vectorStore:mmap
-      // provider is registered AND the storage adapter resolves a real local
-      // path via getBinaryBlobPath(), open the mmap file there and inject the
-      // backend into JsHnswVectorIndex. Cloud adapters return null and skip silently;
-      // JsHnswVectorIndex's behaviour is then identical to pre-2.4.0 (per-entity reads
-      // from canonical storage). Failures are non-fatal — the index still
-      // works, just without the zero-copy fast path.
-      await this.wireMmapVectorBackend()
-
       // Wire the connections codec (2.4.0 #3). When the graph:compression
       // provider is registered AND the metadata index exposes a stable
       // idMapper, inject a codec that encodes HNSW connections as
@@ -9379,72 +9368,6 @@ export class Brainy<T = any> implements BrainyInterface<T> {
     await this.lazyRebuildPromise
   }
 
-  /**
-   * @description Open the mmap-vector backend and inject it into JsHnswVectorIndex.
-   * Called once during init after plugin activation, metadataIndex init, and
-   * graphIndex setup — so the idMapper is available and providers are wired.
-   *
-   * Activation conditions (ALL must hold; otherwise this is a no-op and the
-   * index falls back to per-entity storage reads, the pre-2.4.0 behaviour):
-   * 1. The `vectorStore:mmap` provider is registered (cortex registers this).
-   * 2. The storage adapter resolves a real local path via `getBinaryBlobPath()`
-   *    — cloud adapters return null and the mmap layer is skipped silently.
-   * 3. The metadata index exposes its idMapper (a stable UUID↔int map). The
-   *    mmap layout is keyed by these ints, so 2.4.0 #1 is the prerequisite.
-   *
-   * Vector dimensionality: read from `this.dimensions` if set; otherwise the
-   * default for the standard embedding model (384). The dim is fixed at
-   * file-creation time and cannot change later, so a wrong default would only
-   * matter on the very first init of a brand-new instance. After the first
-   * `add()` the in-memory `this.dimensions` is set, and any subsequent init
-   * (or a re-open) sees the correct value.
-   *
-   * Failures here are non-fatal: a debug-level log records the reason, and
-   * JsHnswVectorIndex continues without the mmap fast path.
-   */
-  private async wireMmapVectorBackend(): Promise<void> {
-    const provider = this.pluginRegistry.getProvider<VectorStoreMmapProvider>('vectorStore:mmap')
-    if (!provider) return
-
-    const storageWithBlob = this.storage as unknown as {
-      getBinaryBlobPath?: (key: string) => string | null
-    }
-    const vectorPath = storageWithBlob.getBinaryBlobPath?.('_vectors/main') ?? null
-    if (!vectorPath) return
-
-    const idMapper = this.metadataIndex.getIdMapper?.()
-    if (!idMapper) return
-
-    const dim = this.dimensions ?? 384
-    const initialCapacity = Math.max(idMapper.size * 2, 1024)
-
-    try {
-      const backend = await MmapVectorBackend.open(
-        provider,
-        vectorPath,
-        dim,
-        initialCapacity,
-        idMapper
-      )
-      this.index.setVectorBackend(backend)
-      if (!this.config.silent) {
-        console.log(
-          `[brainy] vector mmap backend wired (path=${vectorPath}, dim=${dim}, capacity=${initialCapacity})`
-        )
-      }
-    } catch (error) {
-      // Common cases: dim mismatch from a prior init at a different dimension,
-      // permission errors, disk full, file corruption. Index keeps working via
-      // the per-entity storage path; flag the cause for diagnosis.
-      if (!this.config.silent) {
-        console.warn(
-          `[brainy] mmap-vector backend not wired (${(error as Error).message}); ` +
-            `falling back to per-entity vector reads`
-        )
-      }
-    }
-  }
-
   /**
    * @description Wire the HNSW connections codec (2.4.0 #3). Activates when
    * BOTH (a) the `graph:compression` provider is registered (cortex registers