feat: establish TypeScript-aware frontend tooling

tests/unit/promotion-readiness.test.js

@@ -6,7 +6,10 @@ import {
 } from "../../src/application/policies/promotion-readiness.js";
 
 const allGateIds = Object.values(PROMOTION_FORMULA).flat();
-const passing = Object.fromEntries(allGateIds.map((gateId) => [gateId, "PASS"]));
+const passing =
+  /** @type {Record<string, "PASS" | "FAIL" | "UNVERIFIED">} */ (
+    Object.fromEntries(allGateIds.map((gateId) => [gateId, "PASS"]))
+  );
 
 describe("promotion readiness formula", () => {
   it("requires every upstream tier before downstream readiness", () => {
@@ -30,7 +33,11 @@ describe("promotion readiness formula", () => {
       ...passing,
       [failedGate]: "FAIL",
     });
-    expect(result[readiness]).toBe(false);
+    const readinessKey =
+      /** @type {keyof ReturnType<typeof evaluatePromotionReadiness>} */ (
+        readiness
+      );
+    expect(result[readinessKey]).toBe(false);
   });
 
   it("does not treat missing or unverified gates as pass", () => {