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