Housing: Implement multi-floor interior system for staircases

Rooms now have FloorIndex (0=ground, 1+=upper floors) persisted to DB.
Room Z position is calculated from FloorIndex * RoomWmoData.Height
(typically 7 yards per floor). Stairwell rooms (HasStairs flag) are
placed on FloorIndex+1 of the source room.

Changes:
- Room struct: added FloorIndex field
- DB: added floorIndex column to character_housing_rooms
- SpawnRoomMeshObjects: Z = originZ + FloorIndex * ceilingHeight
- HousingRoomEntity: FloorIndex set from room data (was hardcoded 0)
- HandleHousingRoomAdd: stairwell rooms go to FloorIndex+1
- SELECT/INSERT/UPDATE queries updated for floorIndex column
- Client door filtering uses FloorIndex to show/hide stair connections:
  Floor 0: hides downward stairs, Floor >=100: hides upward stairs

sql/updates/characters/master/2026_04_08_01_characters.sql.sql

@@ -0,0 +1 @@
+ALTER TABLE `character_housing_rooms` ADD COLUMN IF NOT EXISTS `floorIndex` INT NOT NULL DEFAULT 0 AFTER `gridY`;