fix: cap cast progress at 100% after descent completion (bug #381)

Defense-in-depth fix across 2 files:

1. combat-descent-actions.ts: Added castProgress: 0 and weaponCastProgress: {}
   to the isDescentComplete set() call so the meter resets immediately.

2. combat-actions.ts: Added isDescentComplete to the early-exit guard in
   processCombatTick so combat processing stops entirely once descent is
   complete, preventing phantom spell casts from stale castProgress values.

All 1196 tests pass.

src/lib/game/stores/combat-actions.ts

@@ -95,7 +95,7 @@ export function processCombatTick(
   const logMessages: string[] = [];
   let totalManaGathered = 0;
 
-  if (state.currentAction !== 'climb') {
+  if (state.currentAction !== 'climb' || state.isDescentComplete) {
     return makeDefaultCombatTickResult(rawMana, elements, state, golemancyState.activeGolems);
   }
 

src/lib/game/stores/combat-descent-actions.ts

@@ -52,7 +52,7 @@ export function advanceRoomOrFloor(get: GetFn, set: SetFn): void {
     get().addActivityLog('floor_transition', `Room ${s.currentRoomIndex + 1} passed`);
 
     if (s.currentFloor <= s.exitFloor && s.currentRoomIndex <= 0) {
-      set({ isDescentComplete: true });
+      set({ isDescentComplete: true, castProgress: 0, weaponCastProgress: {} });
       get().addActivityLog('floor_transition',
         'Descent complete — Exit Spire is now available');
       return;