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/deref_hdr_test.ww

@@ -1,4 +1,4 @@
-// deref_hdr_test — runtime contract for a whole 24B str/slice header loaded
+// Runtime contract for a whole 24B str/slice header loaded
 // BY VALUE through a pointer (`let s = *h` where h:*str / h:*[]T). C1b (#4):
 // the N_UN TK_STAR deref arm fell through to a scalar MOVQ (AX),AX that loaded
 // ONLY word0 (.ptr); .len/.cap were then stored from stale BX/CX, so len(*h)