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/tool/rejects_test.ww

@@ -114,7 +114,6 @@ fn paramshadow_pos(src: str, label: str, want: i32, why: str) void = {
 		"cross-module param"));
 };
 
-// One `ww build` per inline source; the build must exit nonzero.
 fn rejectrows(family: str, labels: []str, srcs: []str) void = {
 	let i: i32 = 0;
 	for (i < labels.len) {