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

src/lib/game/__tests__/combat-actions.test.ts

@@ -26,8 +26,16 @@ function resetStores() {
     spireMode: false,
     currentRoom: { roomType: 'combat', enemies: [] },
     clearedFloors: {},
-    climbDirection: null,
+    climbDirection: 'up',
     isDescending: false,
+    startFloor: 1,
+    exitFloor: 1,
+    currentRoomIndex: 0,
+    roomsPerFloor: 5,
+    descentPeak: null,
+    roomResetState: {},
+    clearedRooms: {},
+    isDescentComplete: false,
     golemancy: { enabledGolems: [], summonedGolems: [], lastSummonFloor: 0 },
     equipmentSpellStates: [],
     comboHitCount: 0,
@@ -103,13 +111,25 @@ describe('processCombatTick', () => {
   });
 
   describe('floor advancement', () => {
-    it('should advance floor when HP reaches 0', () => {
+    it('should advance room when HP reaches 0 (not last room)', () => {
       // Set floor HP very low so it's cleared in one cast
-      useCombatStore.setState({ floorHP: 1, castProgress: 0.99 });
+      // currentRoomIndex=0, roomsPerFloor=5 → clears room 0, advances to room 1
+      useCombatStore.setState({ floorHP: 1, castProgress: 0.99, climbDirection: 'up', currentRoomIndex: 0, roomsPerFloor: 5 });
+      const elements = makeInitialElements(500, {});
+      const result = runCombatTick(1000, elements);
+      // Room advanced, floor stays the same
+      expect(result.currentFloor).toBe(1);
+      // floorHP should be the new room's enemy HP
+      expect(result.floorHP).toBeGreaterThan(0);
+    });
+
+    it('should advance floor when last room on floor is cleared', () => {
+      // Set currentRoomIndex to last room so clearing it advances the floor
+      useCombatStore.setState({ floorHP: 1, castProgress: 0.99, climbDirection: 'up', currentRoomIndex: 4, roomsPerFloor: 5 });
       const elements = makeInitialElements(500, {});
       const result = runCombatTick(1000, elements);
       expect(result.currentFloor).toBe(2);
-      expect(result.floorHP).toBe(getFloorMaxHP(2));
+      expect(result.floorHP).toBeGreaterThan(0);
     });
 
     it('should update maxFloorReached when advancing', () => {