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/defdim_field_test.ww

@@ -1,4 +1,4 @@
-// defdim_field_test — `.len` field-read on an array whose dimension is a
+// `.len` field-read on an array whose dimension is a
 // `def` constant (`[MAX]T`) must resolve the length from the type table, the
 // cgdot sibling of the #21 cgslice fix, migrated from
 // test/wcc/989_defdim_field_run.c (#56). A `def MAX` dim arrives as a