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/set-side.ts

@@ -0,0 +1,54 @@
+import type { DomainEvent } from "./events.js";
+import {
+	type CombatantId,
+	type DomainError,
+	type Encounter,
+	findCombatant,
+	isDomainError,
+} from "./types.js";
+
+export interface SetSideSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+const VALID_SIDES = new Set(["party", "enemy"]);
+
+export function setSide(
+	encounter: Encounter,
+	combatantId: CombatantId,
+	value: "party" | "enemy",
+): SetSideSuccess | DomainError {
+	const found = findCombatant(encounter, combatantId);
+	if (isDomainError(found)) return found;
+
+	if (!VALID_SIDES.has(value)) {
+		return {
+			kind: "domain-error",
+			code: "invalid-side",
+			message: `Side must be "party" or "enemy", got "${value}"`,
+		};
+	}
+
+	const previousSide = found.combatant.side;
+
+	const updatedCombatants = encounter.combatants.map((c) =>
+		c.id === combatantId ? { ...c, side: value } : c,
+	);
+
+	return {
+		encounter: {
+			combatants: updatedCombatants,
+			activeIndex: encounter.activeIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [
+			{
+				type: "SideSet",
+				combatantId,
+				previousSide,
+				newSide: value,
+			},
+		],
+	};
+}