fix(devx): the closure sweep's self-test names its failure instead of crashing

Ablating the sweep predicate (`if (missing.length > 0)` -> `if (false)`) made
the self-test exit 1 through a TypeError on `findings[0].missing`, so the
named case that had already failed never reached the report. The optional
read keeps the exit code and restores the diagnostic.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012zGPuVVX3deAx9LdjK8jCk

Author: claude

scripts/check-dts-closure.mjs

@@ -501,7 +501,7 @@ function selfTest() {
     const m = dualManifest('@fixture/cluster');
     eq(
       'the sweep reports exactly the paths the shared derivation names',
-      sweep(redRoot, [{ dir: 'packages/cluster', manifest: m }]).findings[0].missing.map((x) => x.path),
+      (sweep(redRoot, [{ dir: 'packages/cluster', manifest: m }]).findings[0]?.missing ?? []).map((x) => x.path),
       missingDeclarations(declaredDeclarationPaths(m), () => null).map((x) => x.path),
     );
     eq(