Add combatant side assignment for encounter difficulty

Combatants can now be assigned to party or enemy side via a toggle
in the difficulty breakdown panel. Party-side NPCs subtract their XP
from the encounter total, letting allied NPCs reduce difficulty.
PCs default to party, non-PCs to enemy — users who don't use sides
see no change. Side persists across reload and export/import.

Closes #22

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/application/src/index.ts

@@ -24,6 +24,7 @@ export { setAcUseCase } from "./set-ac-use-case.js";
 export { setCrUseCase } from "./set-cr-use-case.js";
 export { setHpUseCase } from "./set-hp-use-case.js";
 export { setInitiativeUseCase } from "./set-initiative-use-case.js";
+export { setSideUseCase } from "./set-side-use-case.js";
 export { setTempHpUseCase } from "./set-temp-hp-use-case.js";
 export { toggleConcentrationUseCase } from "./toggle-concentration-use-case.js";
 export { toggleConditionUseCase } from "./toggle-condition-use-case.js";

packages/application/src/set-side-use-case.ts

@@ -0,0 +1,18 @@
+import {
+	type CombatantId,
+	type DomainError,
+	type DomainEvent,
+	setSide,
+} from "@initiative/domain";
+import type { EncounterStore } from "./ports.js";
+import { runEncounterAction } from "./run-encounter-action.js";
+
+export function setSideUseCase(
+	store: EncounterStore,
+	combatantId: CombatantId,
+	value: "party" | "enemy",
+): DomainEvent[] | DomainError {
+	return runEncounterAction(store, (encounter) =>
+		setSide(encounter, combatantId, value),
+	);
+}