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/checked.ww

@@ -1,8 +1,7 @@
-// math/checked — overflow-checked integer arithmetic. Ported from
-// Hare's math::checked (ref/hare/math/checked/checked.ha). add*/sub*/
-// mul* return (result, overflow) with wrapping semantics. The saturating
-// (clamp-on-overflow) siblings live in saturating.ww — same `checked`
-// module.
+// Ported from Hare's math::checked (ref/hare/math/checked/checked.ha).
+// add*/sub*/mul* return (result, overflow) with wrapping semantics. The
+// saturating (clamp-on-overflow) siblings live in saturating.ww — same
+// `checked` module.
 //
 // Subset of Hare's surface (Hare splits per type, so these are clean
 // omissions, not divergences):