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/application/src/index.ts

@@ -8,4 +8,5 @@ export { retreatTurnUseCase } from "./retreat-turn-use-case.js";
 export { setAcUseCase } from "./set-ac-use-case.js";
 export { setHpUseCase } from "./set-hp-use-case.js";
 export { setInitiativeUseCase } from "./set-initiative-use-case.js";
+export { toggleConcentrationUseCase } from "./toggle-concentration-use-case.js";
 export { toggleConditionUseCase } from "./toggle-condition-use-case.js";

packages/application/src/toggle-concentration-use-case.ts

@@ -0,0 +1,23 @@
+import {
+	type CombatantId,
+	type DomainError,
+	type DomainEvent,
+	isDomainError,
+	toggleConcentration,
+} from "@initiative/domain";
+import type { EncounterStore } from "./ports.js";
+
+export function toggleConcentrationUseCase(
+	store: EncounterStore,
+	combatantId: CombatantId,
+): DomainEvent[] | DomainError {
+	const encounter = store.get();
+	const result = toggleConcentration(encounter, combatantId);
+
+	if (isDomainError(result)) {
+		return result;
+	}
+
+	store.save(result.encounter);
+	return result.events;
+}