feat: establish TypeScript-aware frontend tooling

tests/fixtures/typecheck/invalid-port-implementation.ts

@@ -0,0 +1,7 @@
+interface ClockPort {
+  now(): number;
+}
+
+export const invalidClock: ClockPort = {
+  now: () => "not-a-number",
+};

tests/fixtures/typecheck/invalid-result-narrowing.ts

@@ -0,0 +1,7 @@
+type Result<T, E> =
+  | Readonly<{ ok: true; value: T }>
+  | Readonly<{ ok: false; error: E }>;
+
+export function invalidUnwrap(result: Result<number, string>): number {
+  return result.value;
+}