fix(vfs): the VFS root never persists a zero-norm vector

A zero-norm vector is lawful inside brainy (cosine distance scores it at
maximum, never a false top hit) but a false attractor for a downstream
engine serving squared-euclidean distance, which cannot tell a real
all-zero vector apart from a legitimate origin point.

- The VFS root now persists with vector [] (the existing "unvectored"
  shape) instead of a real all-zero 384-dim placeholder, and is never
  routed into the deferred-embed pipeline.
- A one-time migration in the root-init path detects a pre-fix store's
  all-zero placeholder root (by norm, not length) and rewrites it to []
  through a new sanctioned Brainy method that keeps the canonical
  vectored-noun ledger honest and removes the row from the vector index.
- The vector-index write seam (AddToVectorIndexOperation,
  ReplaceInVectorIndexOperation, and the generation materializer's direct
  insert) now refuses any real all-zero vector before it reaches a
  provider, loudly naming the entity, while the canonical write still
  lands.
- add()'s dimension-pinning and HNSW-insert gates, and the add-params
  validator, now treat any empty vector as carrying no dimension
  information, closing a latent trap where an explicit `vector: []`
  would have pinned dimensions to 0.

src/storage/adapters/baseStorageAdapter.ts

@@ -1152,6 +1152,24 @@ export abstract class BaseStorageAdapter implements StorageAdapter {
     })
   }
 
+  /**
+   * OPTIONAL narrow ledger hook (see {@link StorageAdapter.noteVectorUnlanded}):
+   * the mirror of {@link noteVectorLanded} — record a noun's vector was just
+   * REMOVED (rewritten to the unvectored `[]` shape). Never below zero: a
+   * caller that (incorrectly) fires this for a noun already unvectored would
+   * otherwise drive the ledger negative — clamped defensively, matching the
+   * delete path's `if (this.totalVectoredNounCount > 0)` guard.
+   * @param id - The noun whose vector was just removed (retained for a
+   *   future narration seam; the count itself needs no id-keyed state).
+   */
+  async noteVectorUnlanded(id: string): Promise<void> {
+    void id
+    if (this.totalVectoredNounCount > 0) this.totalVectoredNounCount--
+    this.scheduleCountPersist().catch(() => {
+      // Ignore persist errors — the in-memory count is authoritative; a later op retries.
+    })
+  }
+
   /**
    * Increment count for entity type - O(1) operation.
    * Concurrency is handled by the process-global mutex