Colorduino plasma demo, matrix test and browser simulator

- ColorduinoPlasma sketch and Colorduino library v1.2.6 (lincomatic @ fff7bd3)
- ColorduinoMatrixTest hardware test sketch
- web/index.html: bit-accurate port of the plasma with large resolutions
- prebuilt 16 MHz firmware (with and without Optiboot)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,4 @@
+build/
+*.elf
+.vscode/
+.claude/