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/utils/distance.ts

@@ -65,6 +65,29 @@ export const cosineDistance: DistanceFunction = (a: Vector, b: Vector): number =
   return 1 - similarity
 }
 
+/**
+ * True when `vector` is a REAL (non-empty) all-zero vector — the "false
+ * attractor" shape this engine's own cosine distance treats safely (a
+ * zero-norm operand always scores the MAXIMUM distance, see
+ * {@link cosineDistance}) but a downstream engine serving squared-euclidean
+ * distance cannot distinguish from a legitimate origin point. THE LAW: a
+ * zero-norm vector is not a vector — it never crosses an engine boundary
+ * (never handed to a vector-index provider as a searchable item).
+ *
+ * A length-0 vector is the UNRELATED "unvectored, not yet embedded" shape
+ * (the deferred-embed stub, a permanently-vectorless system row) and is
+ * deliberately NOT zero-norm here — callers checking for "nothing to index"
+ * should test `vector.length === 0` separately; this only flags the
+ * dangerous non-empty all-zero case.
+ */
+export function isZeroNormVector(vector: readonly number[]): boolean {
+  if (vector.length === 0) return false
+  for (let i = 0; i < vector.length; i++) {
+    if (vector[i] !== 0) return false
+  }
+  return true
+}
+
 /**
  * Calculates the Manhattan (L1) distance between two vectors.
  * Lower values indicate higher similarity.