Replace example shows with numbered production shows and add sparkle flag

- Rename all show .txt files with NNN_ numeric prefix so order is explicit
  and controlled by filename (001_heartbeat_red through 006_party)
- Drop HOME_SHOW special-casing from convert_all.py; show 0 is simply the
  lowest-numbered file
- Add SHOW_FLAG_SPARKLE support: shows can declare '// flags: sparkle' to
  overlay random white flashes on top of the base color each frame
- Wire sparkle into led_controller and config.h (SPARKLE_CHANCE/FRAMES)
- Replace old placeholder/example shows with the six production shows

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

converter/shows/004_pulse_yellow.txt

@@ -0,0 +1,7 @@
+// Yellow pulse — slow breathing effect
+//
+// mode: loop
+
+#202000,    0   // start at dim yellow
+#FFFF00, 1200   // fade up to bright yellow
+#202000, 1200   // fade back down to dim yellow