Adventure Tents native Straw Sleep - Dynamic Sleeping Bag Height

Visual-only behavior change:
- Empty / unused bag body: 0.320 blocks tall.
- Empty / unused pillow: 0.380 blocks tall.
- While the underlying native Straw Bed is occupied: body rises to 0.480 and pillow to 0.530.
- When the player gets up (or the bed is no longer occupied), the shell returns to 0.320 / 0.380.
- Applies to standalone Sleeping Bags and all tent sleeping surfaces.
- Uses the real minecraft:straw_bed occupied block state; no fake-sleep state, teleport, or pose logic was reintroduced.
- Per-tick checks are limited to native sleep cosmetic displays near players. A low-frequency heartbeat cleans up a raised shell if a player disconnects or leaves unusually.
