lib: banner purge + WHY-only comment sweep (rule 8)

Every // ---- section banner dies (132 -> 0): names carry the WHAT.
Narration deleted (filename restatements, run-with lines, what-the-
next-line-does); every ref/hare cite, task cite, divergence, ABI/
layout contract, and ownership qualifier kept (borrowed-view lines
restored where the sweep over-cut). Comment-only proven: all 442
walk-workdir .s and 32 import-probe .s byte-identical before/after;
libbyteid 56-roster all-ID.

lib/math/checked/saturating.ww

@@ -1,5 +1,4 @@
-// math/checked (saturating) — clamp-on-overflow arithmetic. Ported from
-// Hare's math::checked saturating siblings
+// Ported from Hare's math::checked saturating siblings
 // (ref/hare/math/checked/saturating.ha). Part of the same `checked`
 // module as checked.ww. Each sat_* clamps to the type's range on
 // overflow instead of wrapping.