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.

tests/integration/vector-leg-open-build.test.ts

@@ -176,21 +176,22 @@ describe('vector-leg open-build (two-engine gate, last red)', () => {
   })
 
   it('the inverse: only deferred (never-landed) user nouns — the ledger is never inflated by them, and search over them honestly returns []', async () => {
-    // ARCHITECTURAL NOTE (found while building this pin): every brainy store
-    // carries ONE permanent, always-vectored noun beyond user data — the VFS
-    // root (`entities/nouns/.../00000000-0000-0000-0000-000000000000`,
-    // src/vfs/VirtualFileSystem.ts). It is inserted with an explicit all-zero
-    // (but non-empty, length-384) vector on EVERY store's first open — never
-    // deferred (a deliberate WASM-cold-compile-avoidance fix, see that
-    // file's comment) — and VFS init unconditionally re-creates it if
-    // missing, before the rebuild gate ever runs. A literal "0 vectored
-    // nouns" store is therefore unreachable through the public API; a
-    // brand-new store's `vectors.all` floor is 1, not 0. This pin verifies
-    // the law the task names in the ACHIEVABLE form: nouns whose embed is
-    // still deferred/unlanded contribute NOTHING to the vectored-noun ledger
-    // — the coverage-gap comparison sees exactly the root (1), never
-    // root+deferred — and semantic search over deferred-only user content
-    // honestly returns `[]` (no error, no false "coverage restored" claim).
+    // ARCHITECTURAL NOTE (updated by the zero-norm root cure): every brainy
+    // store carries ONE permanent VFS root noun beyond user data
+    // (`entities/nouns/.../00000000-0000-0000-0000-000000000000`,
+    // src/vfs/VirtualFileSystem.ts), created (or, on a pre-fix store,
+    // migrated) on every open — but it is deliberately UNVECTORED (vector
+    // `[]`), never a real all-zero placeholder: a zero-norm vector is not a
+    // vector and never crosses an engine boundary (see that file's
+    // doInitializeRoot() comment). It therefore contributes NOTHING to the
+    // vectored-noun ledger — a brand-new store's `vectors.all` floor is 0,
+    // not 1. This pin verifies the law the task names in the ACHIEVABLE
+    // form: nouns whose embed is still deferred/unlanded contribute NOTHING
+    // to the vectored-noun ledger either — the coverage-gap comparison sees
+    // exactly the baseline (the root, contributing 0), never
+    // baseline+deferred — and semantic search over deferred-only user
+    // content honestly returns `[]` (no error, no false "coverage restored"
+    // claim).
     const dir = mkTmp()
 
     const build: any = new Brainy({
@@ -202,6 +203,8 @@ describe('vector-leg open-build (two-engine gate, last red)', () => {
     })
     await build.init()
     const rootOnlyLedger = await build.storage.getCanonicalCounts()
+    // THE NEW LAW: the root is unvectored — a brand-new store's floor is 0.
+    expect(rootOnlyLedger.vectors.all).toBe(0)
     // Block the embedder permanently so every add below stays deferred and
     // unlanded for the rest of this test (a fast deterministic embedder
     // could otherwise land it before we ever observe the "still 0 extra"