chore(spaces): fix doc comments, design doc accuracy, consistent error style

- Fix doc comment referencing room_to_space instead of space_to_rooms
- Add space_to_rooms forward index to design doc index table
- Use Err! consistently for validation errors in admin commands
- Rename test to follow deserialize_ prefix convention

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/plans/2026-03-17-space-permission-cascading-design.md

@@ -160,9 +160,8 @@ existing `roomid_spacehierarchy_cache`.
 | Space → roles defined        | `com.continuwuity.space.roles`        |
 | Space → user → roles         | `com.continuwuity.space.role.member`  |
 | Space → room → required roles| `com.continuwuity.space.role.room`    |
-| Room → parent Space          | `m.space.child` (reverse lookup) |
-
-The Space → child rooms mapping already exists.
+| Room → parent Spaces         | `m.space.child` (reverse lookup) |
+| Space → child rooms          | `m.space.child` (forward index)  |
 
 ### Cache invalidation triggers
 

src/admin/space/roles.rs

@@ -41,9 +41,7 @@ macro_rules! resolve_space {
 				.await,
 			Ok(ruma::room::RoomType::Space)
 		) {
-			return $self
-				.write_str("Error: The specified room is not a Space.")
-				.await;
+			return Err!("The specified room is not a Space.");
 		}
 		space_id
 	}};
@@ -332,9 +330,7 @@ async fn assign(
 		.unwrap_or_default();
 
 	if !role_defs.roles.contains_key(&role_name) {
-		return self
-			.write_str(&format!("Error: Role '{role_name}' does not exist in this space."))
-			.await;
+		return Err!("Role '{role_name}' does not exist in this space.");
 	}
 
 	let member_event_type = StateEventType::from(SPACE_ROLE_MEMBER_EVENT_TYPE.to_owned());
@@ -438,9 +434,7 @@ async fn require(
 		.unwrap_or_default();
 
 	if !role_defs.roles.contains_key(&role_name) {
-		return self
-			.write_str(&format!("Error: Role '{role_name}' does not exist in this space."))
-			.await;
+		return Err!("Role '{role_name}' does not exist in this space.");
 	}
 
 	let room_event_type = StateEventType::from(SPACE_ROLE_ROOM_EVENT_TYPE.to_owned());

src/core/matrix/space_roles.rs

@@ -176,7 +176,7 @@ mod tests {
 	}
 
 	#[test]
-	fn unknown_fields_ignored() {
+	fn deserialize_ignores_unknown_fields() {
 		let json = r#"{"roles":["nsfw"],"extra_field":"ignored"}"#;
 		let content: SpaceRoleMemberEventContent = serde_json::from_str(json).unwrap();
 		assert_eq!(content.roles, vec!["nsfw"]);

src/service/rooms/roles/mod.rs

@@ -595,9 +595,9 @@ pub async fn sync_power_levels(&self, space_id: &RoomId, room_id: &RoomId) -> Re
 
 /// Auto-join a user to all qualifying child rooms of a Space.
 ///
-/// Iterates over all child rooms in the `room_to_space` reverse index that
-/// belong to the given space, checks whether the user qualifies via their
-/// assigned roles, and force-joins them if they are not already a member.
+/// Iterates over all child rooms in the `space_to_rooms` forward index,
+/// checks whether the user qualifies via their assigned roles, and
+/// force-joins them if they are not already a member.
 #[implement(Service)]
 pub async fn auto_join_qualifying_rooms(
 	&self,
@@ -614,7 +614,7 @@ pub async fn auto_join_qualifying_rooms(
 		return Ok(());
 	}
 
-	// Get all child rooms from the room_to_space reverse index
+	// Get all child rooms via the space_to_rooms forward index
 	let child_rooms = self.get_child_rooms(space_id).await;
 
 	for child_room_id in &child_rooms {