fix: vfs.rename() issues a metadata-only update + rollback of fresh adds removes them

Two fixes reported/found by downstream consumers:

(1) vfs.rename() spread the entire fetched entity into brain.update(),
forwarding a vector field that failed the 384-dimension validation when the
entity was fetched without vectors — every rename threw. A rename is a
path/metadata change, never a content change: the update is now metadata-only
(no vector touched, no re-embedding). Regression test covers the verbatim
consumer repro plus cross-directory moves and EEXIST.

(2) AddToHNSWOperation.itemExists() probed an optional getItem capability
with `await (index as any).getItem?.(id)` — `await undefined` resolves
without throwing, so every item was reported as pre-existing and rollback of
fresh adds never removed them, leaving phantom index entries after a failed
transaction. The probe now feature-detects the method and defaults to false
when absent; update flows pair this op with RemoveFromHNSWOperation whose own
rollback restores the prior vector, so reverse-order rollback reconstructs
the original state either way.

1479/1479 unit suite passing.

tests/unit/vfs/vfs-rename-metadata-only.test.ts

@@ -0,0 +1,68 @@
+/**
+ * @module vfs/vfs-rename-metadata-only.test
+ * @description Regression test for the vfs.rename() vector-dimension crash
+ * (7.31.7). rename() used to spread the entire fetched entity into
+ * brain.update(), forwarding a vector field that failed the 384-dimension
+ * validation when the entity was fetched without vectors. A rename is a
+ * path/metadata change — the update must be metadata-only.
+ *
+ * Production repro (verbatim from the consumer report):
+ *   await brain.vfs.writeFile('/a.txt', 'x')
+ *   await brain.vfs.rename('/a.txt', '/b.txt')   // threw pre-7.31.7
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest'
+import { Brainy } from '../../../src/index.js'
+
+describe('vfs.rename() issues a metadata-only update (7.31.7)', () => {
+  let brain: Brainy
+
+  beforeEach(async () => {
+    brain = new Brainy({ storage: { type: 'memory' } })
+    await brain.init()
+  })
+
+  afterEach(async () => {
+    await brain.close()
+  })
+
+  it('renames a file without touching vectors (the production repro)', async () => {
+    await brain.vfs.writeFile('/a.txt', 'x')
+    await brain.vfs.rename('/a.txt', '/b.txt')
+
+    expect(await brain.vfs.exists('/b.txt')).toBe(true)
+    expect(await brain.vfs.exists('/a.txt')).toBe(false)
+    const content = await brain.vfs.readFile('/b.txt')
+    expect(content.toString()).toBe('x')
+  })
+
+  it('preserves file content and custom metadata across rename', async () => {
+    await brain.vfs.writeFile('/notes/draft.md', 'hello world')
+    await brain.vfs.rename('/notes/draft.md', '/notes/final.md')
+
+    const content = await brain.vfs.readFile('/notes/final.md')
+    expect(content.toString()).toBe('hello world')
+    // The renamed entity's metadata carries the new basename
+    const stat = await brain.vfs.stat('/notes/final.md')
+    expect(stat.size).toBeGreaterThan(0)
+    expect(await brain.vfs.exists('/notes/draft.md')).toBe(false)
+  })
+
+  it('moves a file across directories', async () => {
+    await brain.vfs.mkdir('/src')
+    await brain.vfs.mkdir('/dest')
+    await brain.vfs.writeFile('/src/file.txt', 'moving')
+    await brain.vfs.rename('/src/file.txt', '/dest/file.txt')
+
+    expect(await brain.vfs.exists('/dest/file.txt')).toBe(true)
+    expect(await brain.vfs.exists('/src/file.txt')).toBe(false)
+    const content = await brain.vfs.readFile('/dest/file.txt')
+    expect(content.toString()).toBe('moving')
+  })
+
+  it('throws EEXIST when the target already exists', async () => {
+    await brain.vfs.writeFile('/one.txt', '1')
+    await brain.vfs.writeFile('/two.txt', '2')
+    await expect(brain.vfs.rename('/one.txt', '/two.txt')).rejects.toThrow(/exists/i)
+  })
+})