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/combatStore.ts

@@ -7,11 +7,17 @@ import { createSafeStorage } from '../utils/safe-persist';
 import type { GameAction, SpellState, FloorState, GolemancyState, ActivityLogEntry, AchievementState, EquipmentSpellState, ActivityEventType } from '../types';
 import { getFloorMaxHP } from '../utils';
 import { generateFloorState } from '../utils/room-utils';
-import { generateSpireFloorState } from '../utils/spire-utils';
 import { addActivityLogEntry } from '../utils/activity-log';
 import { processCombatTick, makeInitialSpells } from './combat-actions';
 import { getGuardianForFloor } from '../data/guardian-encounters';
 import type { CombatStore } from './combat-state.types';
+import {
+  enterDescentMode,
+  advanceRoomOrFloor,
+  onEnterRoomDescend,
+  onEnterLibraryRoom,
+  createEnterSpireMode,
+} from './combat-descent-actions';
 
 export const useCombatStore = create<CombatStore>()(
   persist(
@@ -33,6 +39,18 @@ export const useCombatStore = create<CombatStore>()(
       climbDirection: null,
       isDescending: false,
 
+      // ─── Spec: Room navigation state ──────────────────────────────────────
+      startFloor: 1,
+      exitFloor: 1,
+      currentRoomIndex: 0,
+      roomsPerFloor: 1,
+
+      // ─── Spec: Descent tracking state ─────────────────────────────────────
+      descentPeak: null,
+      roomResetState: {},
+      clearedRooms: {},
+      isDescentComplete: false,
+
       // Golemancy
       golemancy: {
         enabledGolems: [],
@@ -158,13 +176,18 @@ export const useCombatStore = create<CombatStore>()(
           currentAction: 'meditate',
           climbDirection: null,
           isDescending: false,
-          currentFloor: 1,
-          floorHP: getFloorMaxHP(1),
-          floorMaxHP: getFloorMaxHP(1),
-          currentRoom: generateFloorState(1),
+          currentFloor: s.exitFloor,
+          floorHP: getFloorMaxHP(s.exitFloor),
+          floorMaxHP: getFloorMaxHP(s.exitFloor),
+          currentRoom: generateFloorState(s.exitFloor),
           castProgress: 0,
           clearedFloors: {},
-          // Preserve maxFloorReached — don't reset to 0 on spire exit (fix #238)
+          clearedRooms: {},
+          roomResetState: {},
+          descentPeak: null,
+          isDescentComplete: false,
+          currentRoomIndex: 0,
+          roomsPerFloor: 1,
           maxFloorReached: Math.max(s.maxFloorReached, 1),
         }));
       },
@@ -174,17 +197,21 @@ export const useCombatStore = create<CombatStore>()(
       startClimbDown: () => set({ climbDirection: 'down', currentAction: 'climb' }),
 
       startPracticing: () => set((s) => {
-        // Only override if the current action is 'meditate' — don't clobber climb/study/etc.
         if (s.currentAction !== 'meditate') return s;
         return { currentAction: 'practicing' };
       }),
 
       stopPracticing: () => set((s) => {
-        // Only restore to meditate if we're currently practicing (don't clobber other actions)
         if (s.currentAction !== 'practicing') return s;
         return { currentAction: 'meditate' };
       }),
 
+      // ─── Spec: Descent actions (delegated to combat-descent-actions.ts) ────
+      enterDescentMode: () => enterDescentMode(get, set),
+      advanceRoomOrFloor: () => advanceRoomOrFloor(get, set),
+      onEnterRoomDescend: () => onEnterRoomDescend(get, set),
+      onEnterLibraryRoom: () => onEnterLibraryRoom(get, set),
+
       // Golemancy
       toggleGolem: (golemId: string) => {
         set((s) => {
@@ -207,22 +234,7 @@ export const useCombatStore = create<CombatStore>()(
         }));
       },
 
-      enterSpireMode: () => {
-        const freshRoom = generateSpireFloorState(1, 0, 1);
-        set((s) => ({
-          spireMode: true,
-          currentAction: 'climb',
-          currentFloor: 1,
-          floorHP: getFloorMaxHP(1),
-          floorMaxHP: getFloorMaxHP(1),
-          currentRoom: freshRoom,
-          castProgress: 0,
-          climbDirection: null,
-          isDescending: false,
-          clearedFloors: {},
-          // Don't inflate maxFloorReached — it should reflect actual progress (fix #238)
-        }));
-      },
+      enterSpireMode: createEnterSpireMode(get, set, generateFloorState),
 
       learnSpell: (spellId: string) => {
         set((state) => ({