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/domain/src/events.ts

@@ -101,6 +101,13 @@ export interface CrSet {
 	readonly newCr: string | undefined;
 }
 
+export interface SideSet {
+	readonly type: "SideSet";
+	readonly combatantId: CombatantId;
+	readonly previousSide: "party" | "enemy" | undefined;
+	readonly newSide: "party" | "enemy";
+}
+
 export interface ConditionAdded {
 	readonly type: "ConditionAdded";
 	readonly combatantId: CombatantId;
@@ -161,6 +168,7 @@ export type DomainEvent =
 	| RoundRetreated
 	| AcSet
 	| CrSet
+	| SideSet
 	| ConditionAdded
 	| ConditionRemoved
 	| ConcentrationStarted