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/arraytoslice_test.ww

@@ -1,4 +1,4 @@
-// arraytoslice_test — #258: the implicit [N]T -> []T array-to-slice BORROW at
+// #258: the implicit [N]T -> []T array-to-slice BORROW at
 // assign / return / call-arg / let init (.ptr = &arr[0], .len = .cap = N).
 // Migrated from test/wcc/953_arraytoslice_run.c (value rows; cs==ww byte-id
 // rides T2). The element-type-mismatch reject rows stay as runww //ww:error