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/canonical-count-ledger.test.ts

@@ -225,9 +225,12 @@ describe('canonical count ledger — the vectored-noun scalar (the vector leg\'s
   }
 
   /** Baseline vectored count right after a fresh open() — init() creates a
-   *  hidden system VFS-root noun that itself carries a real vector, so a
-   *  brand-new store's `vectors.all` is 1, not 0. Tests assert DELTAS off
-   *  this baseline rather than hardcoding it away. */
+   *  hidden system VFS-root noun, but (the zero-norm root cure) it is
+   *  deliberately UNVECTORED (`vector: []`, never a real all-zero
+   *  placeholder — a zero-norm vector never crosses an engine boundary), so
+   *  a brand-new store's `vectors.all` is 0. Tests still assert DELTAS off
+   *  this baseline rather than hardcoding it away, in case that ever
+   *  changes again. */
   let baseline: number
 
   beforeEach(async () => {
@@ -235,6 +238,7 @@ describe('canonical count ledger — the vectored-noun scalar (the vector leg\'s
     dir = fs.mkdtempSync(path.join(os.tmpdir(), 'brainy-vectored-ledger-'))
     brain = await open()
     baseline = (await brain.storage.getCanonicalCounts()).vectors.all
+    expect(baseline).toBe(0) // the unvectored VFS root contributes nothing
   })
   afterEach(async () => {
     vi.restoreAllMocks()
@@ -348,7 +352,7 @@ describe('canonical count ledger — the vectored-noun scalar (the vector leg\'s
 
     brain = await open()
     const ledger = await brain.storage.getCanonicalCounts()
-    expect(ledger.vectors.all).toBe(baseline + 1) // the root + the one non-deferred noun
+    expect(ledger.vectors.all).toBe(baseline + 1) // just the one non-deferred noun — the root is unvectored
     expect(ledger.vectors.all).toBe(countVectoredNouns(dir))
     const persisted = JSON.parse(fs.readFileSync(countsPath, 'utf-8'))
     expect(persisted.totalVectoredNounCount).toBe(baseline + 1)