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/strconv/ftos_test.ww

@@ -1,6 +1,3 @@
-// ftostest — exercises lib/strconv/ftos.ww (Hare ftos.ha / ftos_ryu.ha
-// Ryū port).
-//
 // Ports ref/hare/strconv/+test/ftos_test.ha's ffmt::G / prec=void /
 // fflags::NONE rows (the f64tos cases) verbatim — these are exactly
 // strconv.f64tos's output. encode_f_dec is covered by 13.37/1100/0.011/…