Remove demo signal generator

Audio capture only — shows NO SIGNAL when libav can't connect.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/osc.nim

@@ -66,8 +66,6 @@ proc main() =
         of gsGrid: gsCross
         of gsCross: gsOff
         of gsOff: gsGrid
-    of Key.D:
-      audio.cyclePreset()
     else:
       discard