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_argslice_test.ww

@@ -1,4 +1,4 @@
-// defdim_argslice_test — passing a slice of a `def`-dimensioned array
+// Passing a slice of a `def`-dimensioned array
 // (`take(buf[1:])` on `[MAX]T`) as a call argument must resolve the default-hi
 // length from the type table, the N_SLICE arg-push member of the def-dim
 // family, migrated from test/wcc/989_defdim_argslice_run.c (#56 c4). pushargsrev's