fix(8.0): vfs.rename() issues a metadata-only update (port of the 7.31.7 fix)

rename() spread the entire fetched entity into brain.update(), forwarding a
vector field that failed dimension validation when the entity was fetched
without vectors. A rename is a path/metadata change: the update is now
metadata-only — no vector forwarded, no re-embedding, no vector-index touch.
Regression suite ported from the 7.x line (verbatim consumer repro,
cross-directory move, EEXIST). The companion rollback fix from 7.31.7 was
already present on this branch via the pre-RC1 sweep.

src/vfs/VirtualFileSystem.ts

@@ -1670,17 +1670,6 @@ export class VirtualFileSystem implements IVirtualFileSystem {
       if (err.code !== VFSErrorCode.ENOENT) throw err
     }
 
-    // Update entity metadata
-    const updatedEntity = {
-      ...entity,
-      metadata: {
-        ...entity.metadata,
-        path: newPath,
-        name: this.getBasename(newPath),
-        modified: Date.now()
-      }
-    }
-
     // Update parent relationships if needed
     const oldParentPath = this.getParentPath(oldPath)
     const newParentPath = this.getParentPath(newPath)
@@ -1706,10 +1695,19 @@ export class VirtualFileSystem implements IVirtualFileSystem {
       }
     }
 
-    // Update the entity
+    // A rename is a path/metadata change, never a content change — issue a
+    // metadata-only update. Spreading the whole entity here used to forward
+    // its vector field into update(), which failed dimension validation when
+    // the entity was fetched without vectors (and would needlessly touch the
+    // vector index when it wasn't).
     await this.brain.update({
-      ...updatedEntity,
-      id: entityId
+      id: entityId,
+      metadata: {
+        ...entity.metadata,
+        path: newPath,
+        name: this.getBasename(newPath),
+        modified: Date.now()
+      }
     })
 
     // Update path cache

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)
+  })
+})