fix(spaces): unlink one space-child edge instead of over-deleting (COR-1)

When a single m.space.child was removed (unlinking child C from space P), the
roomToParents reducer fired the whole-room DELETE action, which wiped C's
entire parent set, stripped C as a parent from every other room, and orphaned
C's own descendants until a full resync. So removing C from space A also
dropped C's other parent B, and C's children lost C.

Add a targeted UNLINK {parent, child} action that removes only that one
parent->child edge and prunes the child entry only when its parent set
empties (matching the map's build-time invariant that zero-parent rooms have
no entry). Point the invalid-child branch of handleStateChange at it; DELETE
is unchanged for genuine room leave/delete. Unit-tested (keeps other parents,
prunes on last parent, does NOT orphan descendants, unknown pair no-op).

Verified correct + consumer-safe by two review passes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/state/room/roomToParents.ts

@@ -31,6 +31,11 @@ export type RoomToParentsAction =
   | {
       type: 'DELETE';
       roomId: string;
+    }
+  | {
+      type: 'UNLINK';
+      parent: string;
+      child: string;
     };
 
 const baseRoomToParents = atom<RoomToParents>(new Map());
@@ -63,6 +68,23 @@ export const roomToParentsAtom = atom<RoomToParents, [RoomToParentsAction], unde
           noParentRooms.forEach((room) => draftRoomToParents.delete(room));
         }),
       );
+      return;
+    }
+    if (action.type === 'UNLINK') {
+      // Remove ONE parent from ONE child (a single `m.space.child` was removed).
+      // Unlike DELETE (used when a room is left/deleted), this must NOT touch the
+      // child's other parents, nor strip the child as a parent elsewhere, nor
+      // orphan the child's own descendants. Only prune the child's entry if this
+      // was its last parent.
+      set(
+        baseRoomToParents,
+        produce(get(baseRoomToParents), (draftRoomToParents) => {
+          const parents = draftRoomToParents.get(action.child);
+          if (!parents) return;
+          parents.delete(action.parent);
+          if (parents.size === 0) draftRoomToParents.delete(action.child);
+        }),
+      );
     }
   },
 );
@@ -100,7 +122,10 @@ export const useBindRoomToParentsAtom = (
           if (isValidChild(mEvent)) {
             setRoomToParents({ type: 'PUT', parent: roomId, children: [childId] });
           } else {
-            setRoomToParents({ type: 'DELETE', roomId: childId });
+            // A single m.space.child was removed: unlink only THIS parent→child
+            // edge. DELETE here wiped the child's other parents and orphaned its
+            // descendants until a full resync (COR-1).
+            setRoomToParents({ type: 'UNLINK', parent: roomId, child: childId });
           }
         }
       }