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/lang/tuple_index_read_test.ww

@@ -1,4 +1,4 @@
-// tuple_index_read_test — #121: const-slice tuple-element READ. Migrated from
+// #121: const-slice tuple-element READ. Migrated from
 // test/wcc/947_tuple_index_read_run.c.
 //
 // Three legs of indexed-tuple read off a `const [](str,*fn)` (fold-6's