Persistent presets with visual indicator, plus improved makeup gain

[optilude]

Hi,

I've made the following changes:

• Make the second LED blink periodically to indicate which preset is currently selected.
• Make preset selection persistent across restarts (though it always starts in bypass mode - we could change this)
• Improve the makeup gain calculation to create a more consistent volume output
• Fixed a defect (not sure if I introduced it, or if it was always there) where the pedal would crash due to a race condition after repeatedly cycling presets with all four delay lines on
• Some code consistency/refactoring. I realise this makes the patch pretty large, for which I'm sorry - it was just expedient for me to get my head around everything and implement some of the persistence features as cleanly as I could.

I also added a CLAUDE.md because I used Claude Code to help me with the refactoring. This is obviously not critical to keep. I have reviewed every line of code changed.

I've also tested it myself on my own Terrarium pedal and it seems to work well and sound really nice. Thank you for making and sharing this!

Martin