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/__tests__/toggle-concentration.test.ts

@@ -0,0 +1,69 @@
+import { describe, expect, it } from "vitest";
+import { toggleConcentration } from "../toggle-concentration.js";
+import type { Combatant, Encounter } from "../types.js";
+import { combatantId, isDomainError } from "../types.js";
+
+function makeCombatant(name: string, isConcentrating?: boolean): Combatant {
+	return isConcentrating
+		? { id: combatantId(name), name, isConcentrating }
+		: { id: combatantId(name), name };
+}
+
+function enc(combatants: Combatant[]): Encounter {
+	return { combatants, activeIndex: 0, roundNumber: 1 };
+}
+
+function success(encounter: Encounter, id: string) {
+	const result = toggleConcentration(encounter, combatantId(id));
+	if (isDomainError(result)) {
+		throw new Error(`Expected success, got error: ${result.message}`);
+	}
+	return result;
+}
+
+describe("toggleConcentration", () => {
+	it("toggles concentration on when falsy", () => {
+		const e = enc([makeCombatant("A")]);
+		const { encounter, events } = success(e, "A");
+
+		expect(encounter.combatants[0].isConcentrating).toBe(true);
+		expect(events).toEqual([
+			{ type: "ConcentrationStarted", combatantId: combatantId("A") },
+		]);
+	});
+
+	it("toggles concentration off when true", () => {
+		const e = enc([makeCombatant("A", true)]);
+		const { encounter, events } = success(e, "A");
+
+		expect(encounter.combatants[0].isConcentrating).toBeUndefined();
+		expect(events).toEqual([
+			{ type: "ConcentrationEnded", combatantId: combatantId("A") },
+		]);
+	});
+
+	it("returns error for nonexistent combatant", () => {
+		const e = enc([makeCombatant("A")]);
+		const result = toggleConcentration(e, combatantId("missing"));
+
+		expect(isDomainError(result)).toBe(true);
+		if (isDomainError(result)) {
+			expect(result.code).toBe("combatant-not-found");
+		}
+	});
+
+	it("does not mutate input encounter", () => {
+		const e = enc([makeCombatant("A")]);
+		const original = JSON.parse(JSON.stringify(e));
+		toggleConcentration(e, combatantId("A"));
+		expect(e).toEqual(original);
+	});
+
+	it("does not affect other combatants", () => {
+		const e = enc([makeCombatant("A"), makeCombatant("B", true)]);
+		const { encounter } = success(e, "A");
+
+		expect(encounter.combatants[0].isConcentrating).toBe(true);
+		expect(encounter.combatants[1].isConcentrating).toBe(true);
+	});
+});