Add test coverage for all 17 application layer use cases

Tests verify the get→call→save wiring and error propagation for each
use case. The 15 formulaic use cases share a test file; rollInitiative
and rollAllInitiative have dedicated suites covering their multi-step
logic (creature lookup, modifier calculation, iteration, early return).

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

packages/application/src/__tests__/roll-initiative-use-case.test.ts

@@ -0,0 +1,155 @@
+import {
+	type Creature,
+	type CreatureId,
+	combatantId,
+	createEncounter,
+	creatureId,
+	isDomainError,
+} from "@initiative/domain";
+import { describe, expect, it } from "vitest";
+import { addCombatantUseCase } from "../add-combatant-use-case.js";
+import { rollInitiativeUseCase } from "../roll-initiative-use-case.js";
+import { expectError, requireSaved, stubEncounterStore } from "./helpers.js";
+
+const GOBLIN_ID = creatureId("goblin");
+
+function makeCreature(overrides?: Partial<Creature>): Creature {
+	return {
+		id: GOBLIN_ID,
+		name: "Goblin",
+		source: "mm",
+		sourceDisplayName: "Monster Manual",
+		size: "Small",
+		type: "humanoid",
+		alignment: "neutral evil",
+		ac: 15,
+		hp: { average: 7, formula: "2d6" },
+		speed: "30 ft.",
+		abilities: { str: 8, dex: 14, con: 10, int: 10, wis: 8, cha: 8 },
+		cr: "1/4",
+		initiativeProficiency: 0,
+		proficiencyBonus: 2,
+		passive: 9,
+		...overrides,
+	};
+}
+
+function encounterWithCreatureLink(name: string, creature: CreatureId) {
+	const enc = createEncounter([]);
+	if (isDomainError(enc)) throw new Error("Setup failed");
+	const id = combatantId(name);
+	const store = stubEncounterStore(enc);
+	addCombatantUseCase(store, id, name);
+	const saved = requireSaved(store.saved);
+	const result = createEncounter(
+		saved.combatants.map((c) =>
+			c.id === id ? { ...c, creatureId: creature } : c,
+		),
+		saved.activeIndex,
+		saved.roundNumber,
+	);
+	if (isDomainError(result)) throw new Error("Setup failed");
+	return result;
+}
+
+describe("rollInitiativeUseCase", () => {
+	it("returns domain error when combatant not found", () => {
+		const enc = createEncounter([]);
+		if (isDomainError(enc)) throw new Error("Setup failed");
+		const store = stubEncounterStore(enc);
+
+		const result = rollInitiativeUseCase(
+			store,
+			combatantId("unknown"),
+			10,
+			() => undefined,
+		);
+
+		expectError(result);
+		expect(result.code).toBe("combatant-not-found");
+		expect(store.saved).toBeNull();
+	});
+
+	it("returns domain error when combatant has no creature link", () => {
+		const enc = createEncounter([]);
+		if (isDomainError(enc)) throw new Error("Setup failed");
+		const store1 = stubEncounterStore(enc);
+		addCombatantUseCase(store1, combatantId("Fighter"), "Fighter");
+
+		const store = stubEncounterStore(requireSaved(store1.saved));
+		const result = rollInitiativeUseCase(
+			store,
+			combatantId("Fighter"),
+			10,
+			() => undefined,
+		);
+
+		expectError(result);
+		expect(result.code).toBe("no-creature-link");
+		expect(store.saved).toBeNull();
+	});
+
+	it("returns domain error when creature not found in getter", () => {
+		const enc = encounterWithCreatureLink("Goblin", GOBLIN_ID);
+		const store = stubEncounterStore(enc);
+
+		const result = rollInitiativeUseCase(
+			store,
+			combatantId("Goblin"),
+			10,
+			() => undefined,
+		);
+
+		expectError(result);
+		expect(result.code).toBe("creature-not-found");
+		expect(store.saved).toBeNull();
+	});
+
+	it("calculates initiative from creature and saves", () => {
+		const creature = makeCreature();
+		const enc = encounterWithCreatureLink("Goblin", GOBLIN_ID);
+		const store = stubEncounterStore(enc);
+
+		// Dex 14 -> modifier +2, CR 1/4 -> PB 2, initiativeProficiency 0
+		// So initiative modifier = 2 + 0*2 = 2
+		// Roll 10 + modifier 2 = 12
+		const result = rollInitiativeUseCase(
+			store,
+			combatantId("Goblin"),
+			10,
+			(id) => (id === GOBLIN_ID ? creature : undefined),
+		);
+
+		expect(isDomainError(result)).toBe(false);
+		expect(requireSaved(store.saved).combatants[0].initiative).toBe(12);
+	});
+
+	it("applies initiative proficiency bonus correctly", () => {
+		// CR 5 -> PB 3, dex 16 -> mod +3, initiativeProficiency 1
+		// modifier = 3 + 1*3 = 6, roll 8 + 6 = 14
+		const creature = makeCreature({
+			abilities: {
+				str: 10,
+				dex: 16,
+				con: 10,
+				int: 10,
+				wis: 10,
+				cha: 10,
+			},
+			cr: "5",
+			initiativeProficiency: 1,
+		});
+		const enc = encounterWithCreatureLink("Monster", GOBLIN_ID);
+		const store = stubEncounterStore(enc);
+
+		const result = rollInitiativeUseCase(
+			store,
+			combatantId("Monster"),
+			8,
+			(id) => (id === GOBLIN_ID ? creature : undefined),
+		);
+
+		expect(isDomainError(result)).toBe(false);
+		expect(requireSaved(store.saved).combatants[0].initiative).toBe(14);
+	});
+});