Add graphical disk visualization with real-time sector highlighting

[wjeiv]

Add Real-time Disk Visualization
Replaces the old rectangular BAM grid with a circular floppy disk visualization that shows real-time drive activity.

Changes:

DiskVisualizer.h/cpp - New rendering engine with track/sector layout, BAM coloring, and animated read head
FileBrowser.cpp - Integrated into DisplayDiskInfo() for full disk render on load
main.cpp - Added UpdateHead() hook in UpdateScreen() for real-time head movement
Features:

Concentric rings show D64 track layout (21/19/18/17 sectors per track zone)
BAM-based coloring: green=allocated, black=free, cyan=directory track
Red highlighting for active sector during drive LED activity
Moving read head on south edge with current track number
Integer-only math (no floating point dependencies)
Files Added:

src/DiskVisualizer.h (91 lines)
src/DiskVisualizer.cpp (500+ lines)
Files Modified:

src/FileBrowser.cpp (+20 lines)
src/main.cpp (+5 lines)
See attached MP4 and screenshots for the visual improvements.

Tested on:
Pi Zero 2 W

20260415_193337.mp4

[pottendo]

hi, looks pretty cool - don't know what Steve will do with it;
You may give it a try to merge into my https://github.com/pottendo/pottendo-Pi1541
...though, pls. be aware: users are very sensitive when compatibility is degrading. I found that the whole GFX output impedes real-time responsiveness on the timing critical I/Os and therefore implemented the option to turn off the local Pi output; the average user won't connect a display anyways, so no need to waste cycles. Steve implemented a similar option in his 1.25, which is now also the base for my fork. Compatibility is pretty good... whatever you do pls. test high-end loaders, bitnax, etc. in combination with your code running and check if the system performance remains stable.

Thanks for your contribution in advance, looking forward, pottendo
PS: Steve and myself are loosely collaborating on this project: https://www.lemon64.com/forum/viewtopic.php?p=1075173#p1075173