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/plugin.ts

@@ -285,55 +285,6 @@ export interface GraphCompressionProvider {
   decode(data: Buffer): number[]
 }
 
-/**
- * The `'vectorStore:mmap'` provider — a static factory class for an mmap-backed
- * vector file. Brainy's JsHnswVectorIndex uses the static `.create()` / `.open()`
- * factories to open a single-file store at a derived path on disk-resident
- * storage adapters; non-FS adapters with no local-path support skip the mmap
- * layer and fall back to per-entity vector reads.
- *
- * Cortex registers `NativeMmapVectorStore` as this provider — a Rust mmap'd
- * f32 array with a header, batch read/write APIs, and madvise prefetch. Slot
- * indices are the stable ints from `EntityIdMapper`; lazy migration on read
- * miss writes back from per-entity storage into the slot, converging an
- * upgraded install to the fast path without a big-bang step.
- */
-export interface VectorStoreMmapProvider {
-  /** Create a new vector file with pre-allocated slot capacity. */
-  create(path: string, dim: number, capacity: number): VectorStoreMmapInstance
-  /** Open an existing vector file for read + write. */
-  open(path: string): VectorStoreMmapInstance
-  /** Open an existing vector file for read-only access. */
-  openReadOnly(path: string): VectorStoreMmapInstance
-}
-
-/**
- * An open mmap-vector-store instance — the surface brainy's JsHnswVectorIndex consumes
- * for batch vector reads + lazy migration from per-entity storage on miss.
- */
-export interface VectorStoreMmapInstance {
-  /** Number of vectors written (highest written index + 1). */
-  readonly count: number
-  /** Maximum slot capacity before resize is needed. */
-  readonly capacity: number
-  /** Vector dimensionality. */
-  readonly dim: number
-  /** Write a single vector at `index`. Vector is f64 in JS, stored as f32. */
-  writeVector(index: number, vector: number[]): void
-  /** Write a flat batch starting at `startIndex`. Returns vectors written. */
-  writeVectorsBatch(startIndex: number, vectorsFlat: number[]): number
-  /** Read a single vector at `index`. Returns f64[]. */
-  readVector(index: number): number[]
-  /** Read multiple vectors by index. Returns flat f64[] of length n × dim. */
-  readVectorsBatch(indices: number[]): number[]
-  /** Prefetch via madvise(WILLNEED); no-op on unsupported platforms. */
-  prefetch(indices: number[]): void
-  /** Grow the vector file to a larger slot capacity. Re-maps the file. */
-  resize(newCapacity: number): void
-  /** msync — flush pending writes to disk. */
-  flush(): void
-}
-
 /**
  * Storage adapter factory — plugins register these to provide
  * new storage backends that users reference by name.