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/defdim_slice_test.ww

@@ -1,4 +1,4 @@
-// defdim_slice_test — slicing an array whose dimension is a `def` constant
+// Slicing an array whose dimension is a `def` constant
 // (`[MAX]T`) must resolve the length AND the capacity from the type table, not
 // the AST dimension node, migrated from test/wcc/989_defdim_slice_run.c (#21).
 // A `def MAX` dim arrives as a non-literal node; wwstage's cgslice default-hi