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/unwrap_callarg_slice_test.ww

@@ -1,4 +1,4 @@
-// unwrap_callarg_slice_test — runtime contract for a whole 24B slice header
+// Runtime contract for a whole 24B slice header
 // produced by an UNWRAP source (`mk_sl()!` N_TRYUNW). #6 (Mechanism B, the
 // deep root, BOTH stages, byte-id-BLIND): the unwrap success-shuffle
 // materialised {ptr,len,cap}→{AX,BX,CX} ONLY when the success variant was