test(devx): assert the freshness predicate the walk applies, not a copy of it

The self-test's source-file cases re-spelled `SOURCE_FILE.test(name) &&
!TEST_FILE.test(name)` inline, so they pinned the two constants and nothing
about how `newestSourceMtime` combines them -- an assertion that would stay
green through a walk that had stopped excluding test files. Name the
predicate and call it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KNfj35pXRxnF1D3gZN5zjc

Author: claude

scripts/check-type-check-coverage.mjs

@@ -1413,6 +1413,17 @@ function staleClosure(roots, graph) {
   });
 }
 
+/**
+ * Whether a file name is one a package's `dist/*.d.ts` can be generated from.
+ * Named, rather than inlined into the walk below, so the self-test asserts the
+ * predicate the walk actually applies instead of a copy of it.
+ *
+ * @param {string} name basename
+ */
+function isBuildSource(name) {
+  return SOURCE_FILE.test(name) && !TEST_FILE.test(name);
+}
+
 /**
  * The newest mtime among the TypeScript sources a package's `dist/*.d.ts` is
  * generated from, or 0 when it has none.
@@ -1449,7 +1460,7 @@ function newestSourceMtime(dir) {
         walk(child);
         continue;
       }
-      if (!SOURCE_FILE.test(entry.name) || TEST_FILE.test(entry.name)) continue;
+      if (!isBuildSource(entry.name)) continue;
       const { mtimeMs } = statSync(child);
       if (mtimeMs > newest) newest = mtimeMs;
     }
@@ -2710,7 +2721,7 @@ function selfTest() {
     { label: 'a .json fixture is not', name: 'fixture.json', expect: false },
   ];
   for (const c of sourceFileCases) {
-    const got = SOURCE_FILE.test(c.name) && !TEST_FILE.test(c.name);
+    const got = isBuildSource(c.name);
     if (got !== c.expect) {
       failures.push(`source-file read — ${c.label}: expected ${c.expect}, got ${got}`);
     }