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/002_heartbeat_breathe.txt

@@ -0,0 +1,7 @@
+// Red Heartbeat × 2 then Red Breathing — 2 lub-dub beats, then slow breathing, repeat.
+//
+// mode: loop
+
+// breathing
+#d10e29, 3600   // fade up to bright red
+#050000, 3600   // fade back down to near black