fix(vectors): a zero-norm vector is not a vector, canonical side included, plus the sanctioned unvector door

The engine-pair seam law: a zero-norm vector never crosses an engine
boundary. The index belt already refused to insert one, but the canonical
write and the vectored-noun ledger still counted it, so a near-empty store
whose only vectored row was zero-norm read "1 canonical vectored vs 0
indexed" and threw a not-ready error at open, and a store's own legacy
zero-norm VFS root could trip the same gate before its VFS-init-time cure
ever ran.

- add()/update() (single and transact()) now normalize an explicit
  real all-zero vector to the unvectored [] shape before the dimension
  pin, the ledger flag, and the index ops ever see it (loud, one warn per
  write, canonical write still succeeds).
- The legacy counts.json derivation walk (scanVectoredNounCount) excludes
  a persisted zero-norm row, matching the live ledger's definition.
- A legacy zero-norm VFS root now migrates at open, before the vector-leg
  gate evaluates, via one O(1) fixed-path read (torn-tolerant — skips
  rather than aborting init on a torn root, letting the recovery walk
  heal it) — independent of whether a VirtualFileSystem is ever
  constructed this session.
- update({ id, vector: [] }) (and the same op inside transact()) is now
  the sanctioned, idempotent unvector door: index removal, exactly-once
  ledger decrement, no re-embed, and it clears a pending deferred-embed
  marker rather than leaving it to re-vectorize the row later. The
  combination with deferEmbedding is a typed refusal.
- JsHnswVectorIndex.rebuild() now skips a zero-norm/empty persisted
  vector when repopulating from canonical (the same belt the live
  add/replace paths already had), and health()'s index-parity check now
  compares HNSW size against the vectored-noun ledger rather than the
  raw metadata-entry count, since a store's VFS root is permanently
  unvectored by design.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/hnsw/hnswIndex.ts

@@ -10,7 +10,7 @@ import {
   Vector,
   VectorDocument
 } from '../coreTypes.js'
-import { euclideanDistance, calculateDistancesBatch } from '../utils/index.js'
+import { euclideanDistance, calculateDistancesBatch, isZeroNormVector } from '../utils/index.js'
 import type { BaseStorage } from '../storage/baseStorage.js'
 import { getGlobalCache, UnifiedCache } from '../utils/unifiedCache.js'
 import { prodLog } from '../utils/logger.js'
@@ -1768,6 +1768,33 @@ export class JsHnswVectorIndex implements VectorIndexProvider {
         // Process all nouns at once
         for (const nounData of result.items) {
           try {
+            // THE ZERO-NORM LAW / THE INDEX BELT — bulk-rebuild leg: a row
+            // holding no REAL vector (the "unvectored" `[]` shape — a
+            // deferred embed's stub, or a row the sanctioned unvector door
+            // rewrote) must never enter the index, mirroring the guard
+            // AddToVectorIndexOperation/ReplaceInVectorIndexOperation already
+            // enforce on the live transactional write paths. This matters
+            // HERE specifically: a row can be unvectored (canonical vector
+            // rewritten to `[]`) while its PERSISTED HNSW graph metadata
+            // (`getVectorIndexData` — level/connections) is still present
+            // from before the unvector, so `hnswData`'s mere presence below
+            // is not proof the row belongs in the index — only the
+            // canonical vector itself is authoritative. Checked against
+            // `nounData.vector` (the FULL loaded vector), never the
+            // to-be-truncated `noun.vector` below, so this holds regardless
+            // of the eager/lazy preload strategy chosen further down.
+            if (!Array.isArray(nounData.vector) || nounData.vector.length === 0) {
+              continue
+            }
+            if (isZeroNormVector(nounData.vector)) {
+              prodLog.warn(
+                `[HNSW] rebuild(): skipping entity ${nounData.id} — persisted vector is ` +
+                  `zero-norm (a zero-norm vector is not a vector and never crosses an ` +
+                  `engine boundary)`
+              )
+              continue
+            }
+
             // Load HNSW graph data for this entity
             const hnswData = await this.storage.getVectorIndexData(nounData.id)