refactor(lbug): make the unknown-label node drop explicit (#2215 review)

With the U3 `let pending` switch idiom, a node whose label matches neither
codeWriterMap nor multiLangWriters left `pending` undefined and was silently
dropped — a footgun for a future node type. Add an explicit else with a comment
documenting that unknown labels are intentionally not persisted and that a new
type must be wired into a writer map. No behavior change (byte-identity + tests
unchanged).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

gitnexus/src/core/lbug/csv-generator.ts

@@ -572,6 +572,12 @@ export const streamAllCSVsToDisk = async (
                     : []),
                 ].join(','),
               );
+            } else {
+              // Unknown label: not in codeWriterMap or multiLangWriters, so there
+              // is no CSV table for it and it is intentionally NOT persisted —
+              // `pending` stays undefined, so the loop awaits nothing. Made
+              // explicit so a future node type isn't silently dropped here: wire
+              // it into one of the writer maps above (or this branch).
             }
           }
           break;