Reject impossible K League schedule dates before querying

The kleague-results API previously accepted regex-shaped but impossible calendar dates and treated them like empty-result queries. This change adds a regression test first, then rejects invalid YYYY-MM-DD strings during normalization so bad input never reaches the fetch layer.

Constraint: Public API contract says date must be a valid Date or YYYY-MM-DD string
Rejected: Let upstream return zero matches | hides invalid input as a valid no-results response
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep string-date validation in normalizeDateInput before any schedule fetch
Tested: node --test packages/kleague-results/test/index.test.js; npm run ci; live smoke getKLeagueSummary(2026-03-22, K리그1, FC서울, standings); live smoke getMatchResults(2026-03-22, K리그2); invalid-date smoke getMatchResults(2026-13-40); LSP diagnostics on changed files; architect verification
Not-tested: Additional leap-year and month-end invalid-date permutations

packages/kleague-results/src/parse.js

@@ -65,6 +65,10 @@ function normalizeDateInput(value) {
     throw new Error("date must be a valid Date or YYYY-MM-DD string.");
   }
 
+  if (!isValidCalendarDate(match[1], match[2], match[3])) {
+    throw new Error("date must be a valid Date or YYYY-MM-DD string.");
+  }
+
   return buildDateParts(match[1], match[2], match[3]);
 }
 
@@ -85,6 +89,27 @@ function buildDateParts(year, month, day) {
   };
 }
 
+function isValidCalendarDate(year, month, day) {
+  const numericYear = Number(year);
+  const numericMonth = Number(month);
+  const numericDay = Number(day);
+
+  if (!Number.isInteger(numericYear) || !Number.isInteger(numericMonth) || !Number.isInteger(numericDay)) {
+    return false;
+  }
+
+  if (numericMonth < 1 || numericMonth > 12 || numericDay < 1) {
+    return false;
+  }
+
+  const maxDay = [31, isLeapYear(numericYear) ? 29 : 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31][numericMonth - 1];
+  return numericDay <= maxDay;
+}
+
+function isLeapYear(year) {
+  return year % 4 === 0 && (year % 100 !== 0 || year % 400 === 0);
+}
+
 function buildClubDirectory(clubList = []) {
   const directory = new Map();
 

packages/kleague-results/test/index.test.js

@@ -126,6 +126,24 @@ test("public fetchers compose day results with current standings via mocked fetc
   }
 });
 
+test("getMatchResults rejects impossible calendar dates before fetching", async () => {
+  let fetchCalled = false;
+
+  await assert.rejects(
+    () =>
+      getMatchResults("2026-13-40", {
+        leagueId: "K리그1",
+        fetchImpl: async () => {
+          fetchCalled = true;
+          return makeResponse(schedulePayload);
+        },
+      }),
+    /date must be a valid Date or YYYY-MM-DD string\./,
+  );
+
+  assert.equal(fetchCalled, false);
+});
+
 function makeResponse(body) {
   return new Response(JSON.stringify(body), {
     status: 200,