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/void_error_singleton_test.ww

@@ -1,4 +1,4 @@
-// void_error_singleton_test — a `!void` error singleton (tag-only) across
+// A `!void` error singleton (tag-only) across
 // return / let-init / assign / call-arg positions, plus a wide (str) success
 // payload exercising CX/R8 zeroing on the tag-only return. Migrated from
 // test/wcc/949_void_error_singleton_run.c (value rows; C driver asserted cs==ww .s).