feat: implement Transference Channel system for Enchanter attunement

- Add isChanneling, channelSpeedMultiplier, channelDrainRate to CombatState
- Add startChanneling/stopChanneling actions to combat store
- Add transference-channeling discipline with 3 perks (channel-efficiency, channel-power, channel-mastery)
- Add channelIntensity and channelEfficiency to KNOWN_BONUS_STATS
- Create combat-channel.ts with drain + speed multiplier computation
- Apply channel speed multiplier to equipment spells and melee attacks
- Add Channel Transference hold-button UI to SpireCombatPage
- Add compact channel status indicator to SpireCombatControls
- Channel state resets on spire exit, persists across room transitions
- All 1235 existing tests pass

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

@@ -0,0 +1,20 @@
+// ─── Combat Channel Actions ────────────────────────────────────────────────────
+// Extracted from combatStore.ts to keep the store under the 400-line limit.
+// Provides startChanneling/stopChanneling action factory for the combat store.
+
+import type { CombatStore, CombatState } from './combat-state.types';
+
+type GetFn = () => CombatStore;
+type SetFn = (state: Partial<CombatState>) => void;
+
+export function createChannelActions(get: GetFn, set: SetFn) {
+  return {
+    startChanneling: () => {
+      set({ isChanneling: true });
+    },
+
+    stopChanneling: () => {
+      set({ isChanneling: false });
+    },
+  };
+}