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/union_subtail_bp_test.ww

@@ -1,4 +1,4 @@
-// union_subtail_bp_test — #15: a union-return struct-lit fill stored every
+// #15: a union-return struct-lit fill stored every
 // scalar field with MOVQ except the explicit {1→MOVB, 4→MOVL} arms, so a
 // 2-byte (i16/u16) field fell through to an 8-byte MOVQ. For the LAST field at
 // the frame edge this over-store ran past the slot into saved [BP]: a