test(memory-b): add F8 checkpoint for width validation

scripts/checkpoints.js

@@ -441,6 +441,41 @@ await checkpoint('F7', 'store stats groups active and stale fingerprints with DB
   }
 })
 
+// ── F8: explicit width errors, provider width errors degrade ─────────────
+await checkpoint('F8', 'dimension validation separates caller errors from provider degradation', async () => {
+  const explicitApp = makeApp()
+  const explicit = applyMemory(explicitApp.ctx, {
+    path: join(workspace, 'f8-explicit.db'),
+    embedder: { info: () => ({ fingerprint: 'fp-f8', dimension: 3 }), embedText: async () => [1, 0, 0] },
+  })
+  try {
+    await assert.rejects(
+      () => explicit.commit({ text: 'F8 caller error', vector: [1, 2], fingerprint: 'fp-f8' }),
+      /does not match the active space/,
+    )
+    assert.equal(explicit.store.countBlocks(), 0, 'caller width error must not write a block')
+    await explicit.commit({ text: 'F8 matching caller vector', vector: [1, 2, 3], fingerprint: 'fp-f8' })
+    assert.equal(explicit.store.countBlocks(), 1, 'matching explicit vector is accepted')
+  } finally {
+    await explicitApp.ctx.fiber.dispose()
+  }
+
+  const providerApp = makeApp()
+  const provider = applyMemory(providerApp.ctx, {
+    path: join(workspace, 'f8-provider.db'),
+    embedder: { info: () => ({ fingerprint: 'fp-f8-provider', dimension: 3 }), embedText: async () => [1, 2] },
+  })
+  try {
+    const result = await provider.commit({ text: 'F8 provider error' })
+    assert.equal(result.embedded, false, 'provider width error must degrade without throwing')
+    assert.equal(provider.store.countBlocks(), 1, 'degraded provider write still persists the block')
+    assert.equal(provider.store.allBlocks(1)[0].vector, '', 'the bad provider vector is not stored')
+  } finally {
+    await providerApp.ctx.fiber.dispose()
+  }
+  return 'caller wrong width: rejected/no row; matching width: accepted; provider wrong width: degraded/row retained/vector empty'
+})
+
 // ── cleanup ───────────────────────────────────────────────────────────────
 rmSync(workspace, { recursive: true, force: true })