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/index.ts

@@ -49,6 +49,7 @@ export {
 	editPlayerCharacter,
 } from "./edit-player-character.js";
 export {
+	type CombatantDescriptor,
 	calculateEncounterDifficulty,
 	crToXp,
 	type DifficultyResult,
@@ -75,6 +76,7 @@ export type {
 	PlayerCharacterUpdated,
 	RoundAdvanced,
 	RoundRetreated,
+	SideSet,
 	TempHpSet,
 	TurnAdvanced,
 	TurnRetreated,
@@ -115,6 +117,7 @@ export {
 	type SetInitiativeSuccess,
 	setInitiative,
 } from "./set-initiative.js";
+export { type SetSideSuccess, setSide } from "./set-side.js";
 export { type SetTempHpSuccess, setTempHp } from "./set-temp-hp.js";
 export {
 	type ToggleConcentrationSuccess,