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/utils/paramValidation.ts

@@ -613,6 +613,17 @@ export function validateUpdateParams(params: UpdateParams): void {
   // null/undefined means "no new data was given".
   const hasData = params.data !== undefined && params.data !== null
   if ((params as UpdateParams & { deferEmbedding?: boolean }).deferEmbedding === true) {
+    if (params.vector && params.vector.length === 0) {
+      // The nonsensical combination Leg D of the zero-norm/unvector-door law
+      // refuses: `vector: []` is the SANCTIONED UNVECTOR DOOR — an explicit
+      // instruction to remove the vector NOW, never "please embed" — so it
+      // cannot be paired with a request to defer an embed.
+      throw new Error(
+        `update(): 'vector: []' (the unvector door) cannot be combined with ` +
+          `'deferEmbedding: true' — an unvector is an explicit instruction to remove ` +
+          `the vector now, not a request to defer an embed. Drop one of the two.`
+      )
+    }
     if (params.vector) {
       throw new Error(
         `update(): deferEmbedding cannot be combined with an explicit 'vector' — ` +
@@ -649,8 +660,16 @@ export function validateUpdateParams(params: UpdateParams): void {
     throw new Error(`invalid NounType: ${params.type}`)
   }
   
-  // Validate vector dimensions if provided
-  if (params.vector) {
+  // Validate vector dimensions if provided. A length-0 vector is the
+  // SANCTIONED UNVECTOR DOOR (see brainy.ts update()'s matching comment): an
+  // explicit `vector: []` — or a real all-zero vector, normalized to `[]`
+  // upstream by the zero-norm law — carries no dimension information,
+  // exactly like validateAddParams's identical exemption, so it is exempt
+  // from the dimension check rather than refused as a "0-dimensional
+  // vector". (The `deferEmbedding` combination above already refuses
+  // `vector: []` paired with `deferEmbedding: true` — an empty array is
+  // truthy, so that guard fires unconditionally on any explicit `vector`.)
+  if (params.vector && params.vector.length > 0) {
     const config = ValidationConfig.getInstance()
     if (params.vector.length !== config.maxVectorDimensions) {
       throw new Error(`vector must have exactly ${config.maxVectorDimensions} dimensions`)