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/toggle-concentration.ts

@@ -0,0 +1,44 @@
+import type { DomainEvent } from "./events.js";
+import type { CombatantId, DomainError, Encounter } from "./types.js";
+
+export interface ToggleConcentrationSuccess {
+	readonly encounter: Encounter;
+	readonly events: DomainEvent[];
+}
+
+export function toggleConcentration(
+	encounter: Encounter,
+	combatantId: CombatantId,
+): ToggleConcentrationSuccess | DomainError {
+	const targetIdx = encounter.combatants.findIndex((c) => c.id === combatantId);
+
+	if (targetIdx === -1) {
+		return {
+			kind: "domain-error",
+			code: "combatant-not-found",
+			message: `No combatant found with ID "${combatantId}"`,
+		};
+	}
+
+	const target = encounter.combatants[targetIdx];
+	const wasConcentrating = target.isConcentrating === true;
+
+	const event: DomainEvent = wasConcentrating
+		? { type: "ConcentrationEnded", combatantId }
+		: { type: "ConcentrationStarted", combatantId };
+
+	const updatedCombatants = encounter.combatants.map((c) =>
+		c.id === combatantId
+			? { ...c, isConcentrating: wasConcentrating ? undefined : true }
+			: c,
+	);
+
+	return {
+		encounter: {
+			combatants: updatedCombatants,
+			activeIndex: encounter.activeIndex,
+			roundNumber: encounter.roundNumber,
+		},
+		events: [event],
+	};
+}