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/uniesc_test.ww

@@ -1,4 +1,4 @@
-// uniesc_test — in-language behavior rows for \u / \U / \x escapes,
+// In-language behavior rows for \u / \U / \x escapes,
 // migrated from the retired 110_uniesc_run carrier (task #12
 // proof-of-path; its .wwi round-trip half now lives in
 // test/tool/wwirune_test.ww). The