fix: getNouns().totalCount reports true total, not page size; quiet benign mmap-vector log

getNounsWithPagination returned collectedNouns.length as totalCount, but the
type-first shard scan early-terminates at offset+limit — so
getNouns({ pagination: { limit: 1 } }).totalCount was 1 for any non-empty brain.
The index-rebuild gate calls exactly that, so cold starts logged
"Small dataset (1 items) - rebuilding all indexes" and rebuilt from scratch
regardless of corpus size (a production deployment saw this for an ~8,800-entity
brain). Now reports the authoritative O(1) noun counter (maintained on add/delete,
rehydrated from counts.json on init) as the unfiltered total and derives hasMore
from it. Filtered scans unchanged. Layout-independent (branch/COW included).

Also downgrade the "mmap-vector backend not wired" console.log to prodLog.debug:
it is benign in the native-vector-index model (the native provider owns its own
vector storage and has no setVectorBackend hook), but it fired on every init and
was repeatedly mistaken for the cold-start cause.

Regression: tests/unit/storage/getNouns-totalCount.test.ts. Full unit suite green (1505).

tests/unit/storage/getNouns-totalCount.test.ts

@@ -0,0 +1,88 @@
+/**
+ * @module tests/unit/storage/getNouns-totalCount
+ * @description Regression for BRAINY-MMAP-VECTOR-HOOK (Section H). The
+ * index-rebuild gate (`rebuildIndexesIfNeeded`) reads
+ * `getNouns({ pagination: { limit: 1 } }).totalCount` to decide whether a brain
+ * is "small" enough to rebuild inline. The shard-scan pagination
+ * (`BaseStorage.getNounsWithPagination`) early-terminates at `offset + limit`
+ * for memory efficiency, then returned `collectedNouns.length` as `totalCount` —
+ * i.e. the PAGE size, never the dataset total. So every non-empty filesystem
+ * brain reported `totalCount: 1` and logged "Small dataset (1 items)" on cold
+ * start, regardless of the real corpus size (a consumer saw this for an
+ * ~8.8k-entity brain). `totalCount` must be the authoritative O(1) noun counter
+ * (maintained on every add/delete, rehydrated from `counts.json` on init).
+ */
+import { describe, it, expect } from 'vitest'
+import { mkdtempSync, rmSync } from 'fs'
+import { tmpdir } from 'os'
+import { join } from 'path'
+import { FileSystemStorage } from '../../../src/storage/adapters/fileSystemStorage.js'
+import type { NounMetadata } from '../../../src/coreTypes.js'
+
+const DIM = 8
+
+/** A deterministic non-zero vector so the noun record is well-formed. */
+function vec(seed: number): number[] {
+  return Array.from({ length: DIM }, (_, i) => ((seed + i) % 7) / 7 - 0.5)
+}
+
+/** Storage shards by UUID, so ids must be 32 hex chars. */
+function uuid(i: number): string {
+  return i.toString(16).padStart(32, '0')
+}
+
+/**
+ * Seed `n` unique nouns. Metadata is saved BEFORE the noun record: it is the
+ * recommended order (avoids stat drift) and it is also where the O(1) noun
+ * counter is incremented, so this mirrors the real add() path.
+ */
+async function seed(storage: FileSystemStorage, n: number): Promise<void> {
+  for (let i = 0; i < n; i++) {
+    const id = uuid(i)
+    await storage.saveNounMetadata(id, {
+      noun: 'thing',
+      createdAt: Date.now(),
+      updatedAt: Date.now()
+    } as NounMetadata)
+    await storage.saveNoun({ id, vector: vec(i), connections: new Map(), level: 0 })
+  }
+}
+
+describe('FileSystemStorage.getNouns totalCount (rebuild-gate count)', () => {
+  it('reports the TRUE total, not the page size, for a 1-item page', async () => {
+    const dir = mkdtempSync(join(tmpdir(), 'brainy-totalcount-'))
+    try {
+      const storage = new FileSystemStorage(dir)
+      await storage.init()
+      await seed(storage, 25)
+
+      // The exact call the index-rebuild gate makes.
+      const onePage = await storage.getNouns({ pagination: { limit: 1 } })
+      expect(onePage.items.length).toBe(1)
+      // Before the fix this was 1 (collectedNouns.length === limit).
+      expect(onePage.totalCount).toBe(25)
+      expect(onePage.hasMore).toBe(true)
+    } finally {
+      rmSync(dir, { recursive: true, force: true })
+    }
+  })
+
+  it('still reports the true total after a cold reopen (counts rehydrate)', async () => {
+    const dir = mkdtempSync(join(tmpdir(), 'brainy-totalcount-reopen-'))
+    try {
+      const first = new FileSystemStorage(dir)
+      await first.init()
+      await seed(first, 25)
+      // Flush the count to counts.json so a fresh instance rehydrates it.
+      await (first as unknown as { persistCounts(): Promise<void> }).persistCounts()
+
+      // Cold start: a brand-new instance over the same directory.
+      const reopened = new FileSystemStorage(dir)
+      await reopened.init()
+      const page = await reopened.getNouns({ pagination: { limit: 1 } })
+      expect(page.totalCount).toBe(25)
+    } finally {
+      rmSync(dir, { recursive: true, force: true })
+    }
+  })
+})