Implement the 018-combatant-concentration feature that adds a per-combatant concentration toggle with Brain icon, purple border accent, and damage pulse animation in the encounter tracker

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/domain/src/index.ts

@@ -16,6 +16,8 @@ export type {
 	CombatantAdded,
 	CombatantRemoved,
 	CombatantUpdated,
+	ConcentrationEnded,
+	ConcentrationStarted,
 	ConditionAdded,
 	ConditionRemoved,
 	CurrentHpAdjusted,
@@ -39,6 +41,10 @@ export {
 	type SetInitiativeSuccess,
 	setInitiative,
 } from "./set-initiative.js";
+export {
+	type ToggleConcentrationSuccess,
+	toggleConcentration,
+} from "./toggle-concentration.js";
 export {
 	type ToggleConditionSuccess,
 	toggleCondition,