feat: implement spire descent system with room-aware navigation

Implements the spec-driven spire multi-room climbing and descent system:
- Room navigation: currentRoomIndex, roomsPerFloor, startFloor, exitFloor
- Descent tracking: descentPeak, roomResetState, clearedRooms, isDescentComplete
- enterDescentMode: snapshots peak, sets climbDirection='down'
- advanceRoomOrFloor: room-by-room ascending/descending with floor transitions
- onEnterRoomDescend: per-room 50% reset check with auto-skip
- onEnterLibraryRoom: discipline XP scaled by floor
- Seeded PRNG for deterministic room counts and types
- UI: Descend button during ascent, Exit Spire only when isDescentComplete
- UI: Room X/Y display, room type badge, in-game time in RoomDisplay
- Extracted descent actions to combat-descent-actions.ts (file size limit)
- Updated tests for room-aware combat behavior

Spec: docs/specs/spire-climbing-spec.md §4.1-§4.9, §6

docs/project-structure.txt

@@ -336,6 +336,7 @@ Mana-Loop/
 │   │   │   │   │   └── pact-ritual.ts
 │   │   │   │   ├── attunementStore.ts
 │   │   │   │   ├── combat-actions.ts
+│   │   │   │   ├── combat-descent-actions.ts
 │   │   │   │   ├── combat-state.types.ts
 │   │   │   │   ├── combatStore.ts
 │   │   │   │   ├── crafting-equipment-tick.ts