Hosted rooms' instance IDs name their owner: <owner>-<2100000000+n>

The client takes a room's owner from the instance ID's first part; with 2100000000 there, the owner
couldn't decorate their hosted room (putting furniture on went to their home room instead).

Co-Authored-By: Claude Opus 5.5 <[email protected]>

docs/api.md

@@ -65,12 +65,12 @@ the `GET`s also signed out.
 | `DELETE /api/v1/hosted-rooms/:n` | (owner or moderator) closes it for good; everyone in it gets `room.closed` |
 
 A room is `{n, instanceId, chatId, name, description, roomPid, rating, maxUsers, ownerImvuId, ownerName, occupancy,
-createdAt}`; `instanceId` is `2100000000-<n>`, `chatId` 2100000000 + n, `occupancy` the people in it now.
+createdAt}`; `instanceId` is `<owner IMVU ID>-<2100000000 + n>`, `chatId` 2100000000 + n, `occupancy` the people in it now.
 
 ## Decorations
 
 T5 products placed in ordinary IMVU rooms (docs/products.md, "Decorations"). `instance` is IMVU's room instance,
-`<owner's customer ID>-<n>` (`^[1-9][0-9]{0,11}-[1-9][0-9]{0,9}$`); hosted rooms' (`2100000000-<n>`) are refused
+`<owner's customer ID>-<n>` (`^[1-9][0-9]{0,11}-[1-9][0-9]{0,9}$`); hosted rooms' (room number 2100000000 and up) are refused
 (400 `hosted_room`: they have their own scene). A room's decorations are `{instance, contents, hash}`: `contents`
 is a list of `[furniture instance ID (integer), hub product ID, placement (up to 200 characters)]`, `hash` the
 sha256 (hex) of `contents` as JSON.