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/arrlit_elem_narrow_test.ww

@@ -1,4 +1,4 @@
-// arrlit_elem_narrow_test — #251: narrow int/rune array-literal elements to a
+// #251: narrow int/rune array-literal elements to a
 // declared [N]T element type at the let / def / struct-field init sites.
 // Migrated from test/wcc/951_arrlit_elem_narrow_run.c (accept rows; cs==ww
 // byte-id rides T2). The out-of-range and str-elem reject rows stay as runww