feat(session): open barrels, ender chests and shulker boxes

[xRookieFight]

Description

Only a chest opened. Barrels, trapped chests, shulker boxes and ender chests were registered blocks that did nothing when clicked, and the storage path was written around the one block that worked.

• A block that stores items now declares it once, through block.container_kind. Adding a container is a registration rather than another branch in the session.
• Barrels, trapped chests and every shulker box colour open, store and drop their contents on break, on the same world-backed path the chest already used.
• Ender chests store on the player instead of the block: every ender chest shows the same 27 slots, nothing drops when one is broken, and the contents save and load with the rest of the player data.
• The session's open-container tracking carries what kind is open rather than just a position, so the read, write and release paths ask the kind instead of assuming a chest.

Related issue

Part of #130

Double chests are not in this PR. Pairing is not a server-side decision the client will follow: it reads pairx/pairz off the two chest block entities, and without writing those the client draws 27 slots however many the server offers - which would leave half a double chest unreachable. It needs chest block-entity data first.

Shulker boxes also still drop their contents like any other container rather than keeping them in the item, which needs item NBT round-tripping (#120).

Checklist

• v -check . is clean
• v test server/block, server/player, and the session inventory/items/blocks/place/break tests are green
• Follows the conventions in AGENTS.md (OOP, pub/capitalized exports, no import cycles, minimal comments)
• Cross-session gameplay state is only mutated on its owning world's actor thread (via world_call/wr.submit/WorldTx), never through a global Hub actor
• No unrelated changes bundled in