Add manual CR assignment and difficulty breakdown panel

Implement issue #21: custom combatants can now have a challenge rating
assigned via a new breakdown panel, opened by tapping the difficulty
indicator. Bestiary-linked combatants show read-only CR with source name;
custom combatants get a CR picker with all standard 5e values. CR persists
across reloads and round-trips through JSON export/import.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/web/src/components/__tests__/difficulty-breakdown-panel.test.tsx

@@ -0,0 +1,232 @@
+// @vitest-environment jsdom
+import "@testing-library/jest-dom/vitest";
+
+import type { Creature, CreatureId, PlayerCharacter } from "@initiative/domain";
+import { combatantId, creatureId, playerCharacterId } from "@initiative/domain";
+import { cleanup, render, screen, waitFor } from "@testing-library/react";
+import userEvent from "@testing-library/user-event";
+import { afterEach, beforeAll, describe, expect, it, vi } from "vitest";
+import { createTestAdapters } from "../../__tests__/adapters/in-memory-adapters.js";
+import {
+	buildCombatant,
+	buildCreature,
+	buildEncounter,
+} from "../../__tests__/factories/index.js";
+import { AllProviders } from "../../__tests__/test-providers.js";
+import { DifficultyBreakdownPanel } from "../difficulty-breakdown-panel.js";
+
+beforeAll(() => {
+	Object.defineProperty(globalThis, "matchMedia", {
+		writable: true,
+		value: vi.fn().mockImplementation((query: string) => ({
+			matches: false,
+			media: query,
+			onchange: null,
+			addListener: vi.fn(),
+			removeListener: vi.fn(),
+			addEventListener: vi.fn(),
+			removeEventListener: vi.fn(),
+			dispatchEvent: vi.fn(),
+		})),
+	});
+});
+
+afterEach(cleanup);
+
+const pcId1 = playerCharacterId("pc-1");
+const goblinCreature = buildCreature({
+	id: creatureId("srd:goblin"),
+	name: "Goblin",
+	cr: "1/4",
+	source: "srd",
+	sourceDisplayName: "SRD",
+});
+
+function renderPanel(options: {
+	encounter: ReturnType<typeof buildEncounter>;
+	playerCharacters?: PlayerCharacter[];
+	creatures?: Map<CreatureId, Creature>;
+	onClose?: () => void;
+}) {
+	const adapters = createTestAdapters({
+		encounter: options.encounter,
+		playerCharacters: options.playerCharacters ?? [],
+		creatures: options.creatures,
+	});
+	return render(
+		<AllProviders adapters={adapters}>
+			<DifficultyBreakdownPanel onClose={options.onClose ?? (() => {})} />
+		</AllProviders>,
+	);
+}
+
+function defaultEncounter() {
+	return buildEncounter({
+		combatants: [
+			buildCombatant({
+				id: combatantId("c-1"),
+				name: "Hero",
+				playerCharacterId: pcId1,
+			}),
+			buildCombatant({
+				id: combatantId("c-2"),
+				name: "Goblin",
+				creatureId: goblinCreature.id,
+			}),
+			buildCombatant({
+				id: combatantId("c-3"),
+				name: "Custom Thug",
+				cr: "2",
+			}),
+			buildCombatant({
+				id: combatantId("c-4"),
+				name: "Bandit",
+			}),
+		],
+	});
+}
+
+const defaultPCs: PlayerCharacter[] = [
+	{ id: pcId1, name: "Hero", ac: 15, maxHp: 30, level: 5 },
+];
+
+describe("DifficultyBreakdownPanel", () => {
+	it("renders party budget section", async () => {
+		renderPanel({
+			encounter: defaultEncounter(),
+			playerCharacters: defaultPCs,
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		await waitFor(() => {
+			expect(
+				screen.getByText("Party Budget", { exact: false }),
+			).toBeInTheDocument();
+			expect(screen.getByText("1 PC", { exact: false })).toBeInTheDocument();
+			expect(screen.getByText("Low:", { exact: false })).toBeInTheDocument();
+		});
+	});
+
+	it("renders tier label", async () => {
+		renderPanel({
+			encounter: defaultEncounter(),
+			playerCharacters: defaultPCs,
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		await waitFor(() => {
+			expect(
+				screen.getByText("Encounter Difficulty:", { exact: false }),
+			).toBeInTheDocument();
+		});
+	});
+
+	it("renders bestiary combatant as read-only with source name", async () => {
+		renderPanel({
+			encounter: defaultEncounter(),
+			playerCharacters: defaultPCs,
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		await waitFor(() => {
+			expect(screen.getByText("Goblin (SRD)")).toBeInTheDocument();
+			expect(screen.getAllByText("CR 1/4").length).toBeGreaterThanOrEqual(1);
+		});
+	});
+
+	it("renders custom combatant with CR picker", async () => {
+		renderPanel({
+			encounter: defaultEncounter(),
+			playerCharacters: defaultPCs,
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		await waitFor(() => {
+			const pickers = screen.getAllByLabelText("Challenge rating");
+			expect(pickers).toHaveLength(2);
+			// First picker is "Custom Thug" with CR 2
+			expect(pickers[0]).toHaveValue("2");
+		});
+	});
+
+	it("renders unassigned combatant with Assign picker and dash for XP", async () => {
+		renderPanel({
+			encounter: defaultEncounter(),
+			playerCharacters: defaultPCs,
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		await waitFor(() => {
+			const pickers = screen.getAllByLabelText("Challenge rating");
+			// Second picker is "Bandit" with no CR
+			expect(pickers[1]).toHaveValue("");
+			// "—" appears for unassigned XP
+			expect(screen.getByText("—")).toBeInTheDocument();
+		});
+	});
+
+	it("selecting a CR updates the visible XP value", async () => {
+		const user = userEvent.setup();
+		renderPanel({
+			encounter: defaultEncounter(),
+			playerCharacters: defaultPCs,
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		// Wait for the panel to render with bestiary data
+		await waitFor(() => {
+			expect(screen.getByText("—")).toBeInTheDocument();
+		});
+
+		// The Bandit (second picker) has no CR — shows "—" for XP
+		const pickers = screen.getAllByLabelText("Challenge rating");
+
+		// Select CR 5 (1,800 XP) on Bandit
+		await user.selectOptions(pickers[1], "5");
+
+		// XP should update — the "—" should be replaced with an XP value
+		await waitFor(() => {
+			expect(screen.getByText("1,800")).toBeInTheDocument();
+		});
+	});
+
+	it("renders total monster XP", async () => {
+		renderPanel({
+			encounter: defaultEncounter(),
+			playerCharacters: defaultPCs,
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+		});
+
+		await waitFor(() => {
+			expect(screen.getByText("Total Monster XP")).toBeInTheDocument();
+		});
+	});
+
+	it("renders nothing when breakdown data is insufficient", () => {
+		// No PCs with level → breakdown returns null
+		const { container } = renderPanel({
+			encounter: buildEncounter({
+				combatants: [
+					buildCombatant({ id: combatantId("c-1"), name: "Custom" }),
+				],
+			}),
+		});
+
+		expect(container.innerHTML).toBe("");
+	});
+
+	it("calls onClose when Escape is pressed", async () => {
+		const user = userEvent.setup();
+		const onClose = vi.fn();
+		renderPanel({
+			encounter: defaultEncounter(),
+			playerCharacters: defaultPCs,
+			creatures: new Map([[goblinCreature.id, goblinCreature]]),
+			onClose,
+		});
+
+		await user.keyboard("{Escape}");
+
+		expect(onClose).toHaveBeenCalledOnce();
+	});
+});