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__/helpers.ts

@@ -0,0 +1,54 @@
+import type { Encounter, PlayerCharacter } from "@initiative/domain";
+import { isDomainError } from "@initiative/domain";
+import type { EncounterStore, PlayerCharacterStore } from "../ports.js";
+
+export function requireSaved<T>(value: T | null): T {
+	if (value === null) throw new Error("Expected store.saved to be non-null");
+	return value;
+}
+
+export function expectSuccess<T>(
+	result: T,
+): asserts result is Exclude<T, { kind: "domain-error" }> {
+	if (isDomainError(result)) {
+		throw new Error(`Expected success, got domain error: ${result.message}`);
+	}
+}
+
+export function expectError(result: unknown): asserts result is {
+	kind: "domain-error";
+	code: string;
+	message: string;
+} {
+	if (!isDomainError(result)) {
+		throw new Error("Expected domain error");
+	}
+}
+
+export function stubEncounterStore(
+	initial: Encounter,
+): EncounterStore & { saved: Encounter | null } {
+	const stub = {
+		saved: null as Encounter | null,
+		get: () => initial,
+		save: (e: Encounter) => {
+			stub.saved = e;
+			stub.get = () => e;
+		},
+	};
+	return stub;
+}
+
+export function stubPlayerCharacterStore(
+	initial: readonly PlayerCharacter[],
+): PlayerCharacterStore & { saved: readonly PlayerCharacter[] | null } {
+	const stub = {
+		saved: null as readonly PlayerCharacter[] | null,
+		getAll: () => [...initial],
+		save: (characters: PlayerCharacter[]) => {
+			stub.saved = characters;
+			stub.getAll = () => [...characters];
+		},
+	};
+	return stub;
+}