T012–T016: Phase 3 application + web shell (use case, ports, React hook, UI, README)

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

packages/application/src/advance-turn-use-case.ts

@@ -0,0 +1,21 @@
+import {
+	advanceTurn,
+	type DomainError,
+	type DomainEvent,
+	isDomainError,
+} from "@initiative/domain";
+import type { EncounterStore } from "./ports.js";
+
+export function advanceTurnUseCase(
+	store: EncounterStore,
+): DomainEvent[] | DomainError {
+	const encounter = store.get();
+	const result = advanceTurn(encounter);
+
+	if (isDomainError(result)) {
+		return result;
+	}
+
+	store.save(result.encounter);
+	return result.events;
+}