chore: import upstream snapshot with attribution

tests/unit/check-type-coverage.test.ts

@@ -0,0 +1,78 @@
+import { test } from "node:test";
+import assert from "node:assert";
+import {
+  parseTypeCoverageOutput,
+  evaluateTypeCoverage,
+} from "../../scripts/check/check-type-coverage.mjs";
+
+// The .mjs module has no .d.ts; type the exported functions locally.
+type TypeCoverageVerdict = { regressed: boolean; improved: boolean };
+
+const parse = parseTypeCoverageOutput as (jsonText: string) => number;
+const evaluate = evaluateTypeCoverage as (
+  current: number,
+  baseline: number,
+  eps?: number
+) => TypeCoverageVerdict;
+
+// --- parseTypeCoverageOutput tests ---
+
+test("parseTypeCoverageOutput: parses valid JSON with percent field", () => {
+  const pct = parse(JSON.stringify({ percent: 92.17, atLeast: 90 }));
+  assert.equal(pct, 92.17);
+});
+
+test("parseTypeCoverageOutput: throws on invalid JSON", () => {
+  assert.throws(() => parse("not json"), /Failed to parse JSON output/);
+});
+
+test("parseTypeCoverageOutput: throws if percent field is missing", () => {
+  assert.throws(() => parse(JSON.stringify({ atLeast: 90 })), /missing numeric 'percent' field/);
+});
+
+test("parseTypeCoverageOutput: throws if percent is not a number", () => {
+  assert.throws(
+    () => parse(JSON.stringify({ percent: "92.17" })),
+    /missing numeric 'percent' field/
+  );
+});
+
+// --- evaluateTypeCoverage tests (direction = up: drops are regressions) ---
+
+const FROZEN = 92.17;
+const EPS = 0.05; // small tolerance for float noise
+
+test("evaluateTypeCoverage: equal to baseline passes (within eps)", () => {
+  const r = evaluate(FROZEN, FROZEN, EPS);
+  assert.equal(r.regressed, false);
+  assert.equal(r.improved, false);
+});
+
+test("evaluateTypeCoverage: drop beyond eps is a regression", () => {
+  const r = evaluate(FROZEN - 0.1, FROZEN, EPS);
+  assert.equal(r.regressed, true);
+  assert.equal(r.improved, false);
+});
+
+test("evaluateTypeCoverage: drop within eps is not a regression", () => {
+  const r = evaluate(FROZEN - 0.03, FROZEN, EPS);
+  assert.equal(r.regressed, false);
+});
+
+test("evaluateTypeCoverage: improvement passes and is flagged", () => {
+  const r = evaluate(FROZEN + 1, FROZEN, EPS);
+  assert.equal(r.regressed, false);
+  assert.equal(r.improved, true);
+});
+
+test("evaluateTypeCoverage: 100% coverage is maximum improvement", () => {
+  const r = evaluate(100, FROZEN, EPS);
+  assert.equal(r.regressed, false);
+  assert.equal(r.improved, true);
+});
+
+test("evaluateTypeCoverage: no eps defaults to 0 (exact comparison)", () => {
+  // With eps=0, even a tiny drop regresses
+  const r = evaluate(FROZEN - 0.01, FROZEN);
+  assert.equal(r.regressed, true);
+});