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/stores/combat-actions.ts

@@ -128,19 +128,27 @@ export function processCombatTick(
       castProgress -= 1;
       safetyCounter++;
 
-      // Check if floor is cleared
+      // Check if room/floor is cleared
       if (floorHP <= 0) {
         const guardian = getGuardianForFloor(currentFloor);
         onFloorCleared(currentFloor, !!guardian);
-        currentFloor = Math.min(currentFloor + 1, 100);
-        floorMaxHP = getFloorMaxHP(currentFloor);
-        floorHP = floorMaxHP;
+
+        // ── Spec: room-aware advancement (climbing spec §4.4) ──────────────
+        // Instead of directly incrementing the floor, delegate to the store's
+        // advanceRoomOrFloor which handles room-by-room and floor transitions.
+        get().advanceRoomOrFloor();
+
+        // Re-read state after advancement
+        const newState = get();
+        currentFloor = newState.currentFloor;
+        floorMaxHP = newState.floorMaxHP;
+        floorHP = newState.floorHP;
         castProgress = 0;
 
         if (guardian) {
           logMessages.push(`\u2694\ufe0f ${guardian.name} defeated!`);
         } else if (currentFloor % 5 === 0) {
-          logMessages.push(`🏰 Floor ${currentFloor - 1} cleared!`);
+          logMessages.push(`\ud83c\udff0 Floor ${currentFloor - 1} cleared!`);
         }
       }
     }
@@ -197,14 +205,19 @@ export function processCombatTick(
         if (floorHP <= 0) {
           const eGuardian = getGuardianForFloor(currentFloor);
           onFloorCleared(currentFloor, !!eGuardian);
-          currentFloor = Math.min(currentFloor + 1, 100);
-          floorMaxHP = getFloorMaxHP(currentFloor);
-          floorHP = floorMaxHP;
+
+          // ── Spec: room-aware advancement ─────────────────────────────────
+          get().advanceRoomOrFloor();
+
+          const newState = get();
+          currentFloor = newState.currentFloor;
+          floorMaxHP = newState.floorMaxHP;
+          floorHP = newState.floorHP;
           eCastProgress = 0;
           if (eGuardian) {
             logMessages.push(`\u2694\ufe0f ${eGuardian.name} defeated!`);
           } else if (currentFloor % 5 === 0) {
-            logMessages.push(`🏰 Floor ${currentFloor - 1} cleared!`);
+            logMessages.push(`\ud83c\udff0 Floor ${currentFloor - 1} cleared!`);
           }
           break;
         }