refactor(rust): collapse save layer and trim unused APIs

[h2cone]

Summary

Collapse the unused save facade into core::progress and delete room-session helper modules that only forwarded work. This is mostly a code-size cut: 40 files, +193 / -2349.

Gameplay impact

• Continue / death still use a single in-memory checkpoint. Unused save-slot APIs are gone.
• The player aim indicator is removed from player.tscn.
• World map no longer handles click-to-select; mouse events pass through.
• Pressure plates resolve their door only via LDtk target_id, not target_room.
• Unused Godot-facing entity/UI methods and signals (checkpoint, key/lock, crate freeze, door toggle, etc.) are gone.

What changed

• core::progress owns the process-local store directly; save/ is deleted.
• player_spawner, portal_connector, and room_event_connector are inlined into GameRoomManager / PlayerRuntime.
• Player input and animation use fixed action/animation names instead of unused config structs.
• Room size is the existing ROOM_WIDTH / ROOM_HEIGHT constants.
• Unused glicol-exporter crate and a leftover .depren tileset file are deleted.
• Docs (AGENTS.md, ARCHITECTURE.md) match the new module map.

Test plan

• cd rust; cargo test --locked — 93 passed
• New Game resets progress and starts at the initial room
• Continue loads the last checkpoint when one exists
• Death with a checkpoint respawns there; death without one restarts
• Stars, keys, locks, and explored rooms persist across room loads
• Pressure plates still open their linked switch doors
• World map still highlights the current / explored rooms