feat(session): snapshot getter for UserDataManager room user list

Extends the snapshot pattern to the room's user list. The React client
currently has many widgets each calling `getUserDataByIndex(idx)` in a
loop (chat, doorbell, room player list, infostand …) — every render
walks the underlying Map and rebuilds an array. With
`getRoomUserListSnapshot(): ReadonlyArray<IRoomUserData>` consumers can
memoize on the array reference and only rebuild when something actually
changed.

Invalidation fires on every state-changing path:
- updateUserData (add/replace)
- removeUserData (leave)
- updateFigure / updateName / updateMotto / updateNickIcon /
  updateCustomization / updateBackground / updateAchievementScore /
  updatePetLevel / updatePetBreedingStatus

The inner IRoomUserData objects keep their existing in-place mutation
semantics (deep-clone would be too expensive for 30+ avatars on every
single status push). Consumers should treat each entry as a
snapshot-at-time-of-read and not retain references across an
invalidation.

New event: NitroEventType.ROOM_USER_LIST_UPDATED. Interface and event
additions are backwards-compatible; no existing accessors changed.

Also tidied: `updatePetLevel` now uses the explicit `if(!userData)
return;` guard pattern matching the rest of the methods (was a one-line
inline conditional).

packages/events/src/NitroEventType.ts

@@ -21,4 +21,5 @@ export class NitroEventType
     public static readonly ROOM_SESSION_UPDATED = 'ROOM_SESSION_UPDATED';
     public static readonly IGNORED_USERS_UPDATED = 'IGNORED_USERS_UPDATED';
     public static readonly GROUP_BADGES_UPDATED = 'GROUP_BADGES_UPDATED';
+    public static readonly ROOM_USER_LIST_UPDATED = 'ROOM_USER_LIST_UPDATED';
 }