driveAtlas: cut ANSI escape churn in the ZimaBoard heatsink

The fin ramp alternated colour keys every two glyphs (█▓ bright, ▒░ mid),
so the run-length colour encoder emitted a new escape roughly every 2.6
characters: 25 escapes per fin line, inflating a 66-glyph line to 334
bytes and 206 escapes for the drawing as a whole.

The █▓▒░ ramp already carries the fin relief through glyph ink density,
so alternating colour across it adds nothing visible. Paint the whole fin
field one steel key instead: fin lines drop to 5 escapes / 114 bytes and
the drawing to 134 escapes total, with byte-identical monochrome output
and no visual change in colour. Gentler on terminals that cope poorly
with dense escape/multibyte interleaving.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

driveAtlas.sh

@@ -1375,8 +1375,8 @@ generate_zimaboard_layout() {
     # heatsink fin ramp: period-4 ridge, lit from upper-left
     _zfin() {
         case $(( (($1%4)+4)%4 )) in
-            0) _zp "$2" "$3" '█' H ;;
-            1) _zp "$2" "$3" '▓' H ;;
+            0) _zp "$2" "$3" '█' S ;;
+            1) _zp "$2" "$3" '▓' S ;;
             2) _zp "$2" "$3" '▒' S ;;
             3) _zp "$2" "$3" '░' S ;;
         esac