test: banner purge + WHY-only comment sweep (rule 8)

Every section banner dies (103 -> 0) across test/lang, the observer
suites, the C carriers, and the five comment-heavy corpus fixtures;
banner provenance (#N cites, carrier numbers, repair-cluster labels)
folded into headers or adjacent WHY comments. Narration deleted; row
provenance, ref cites, divergence pins, and layout contracts kept
(fwd-ref decl-order guards and bootstrap-gate corpus rationale
restored where the sweep over-cut). Comment-only proven: all 3742
wwbuild workdir .s byte-identical before/after; test-commit and
test-byteid (161 lang + 1399 data, 0 pinned-divergent) green.

test/wcc/400_w6c.c

@@ -1,6 +1,5 @@
 /*
- * 400_w6c — in-process codegen smoke tests.  Each row supplies one tiny ww
- * source and the assembly fragments it must emit.  Full-file golden matches
+ * Rows match assembly fragments, not full-file goldens: golden matches
  * would bind the test to unrelated output details.
  */
 #include "gc.h"