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/subslice_cap_test.ww

@@ -1,4 +1,4 @@
-// subslice_cap_test — a sub-slice `base[lo:hi]` must set its capacity word to
+// A sub-slice `base[lo:hi]` must set its capacity word to
 // base_cap - lo (storage remaining to the underlying end; Go/Hare-identical),
 // NOT hi - lo (the new length), migrated from test/wcc/942_subslice_cap_run.c
 // (task #20). base_cap is the array length N for [N]T, or the carried .capacity