Add Happy Hare MMU integration and Docker build tools

[boetti-dev]

Summary

This PR adds Happy Hare MMU support to COSMOS firmware for the Elegoo Centauri Carbon, enabling multi-material printing with aftermarket MMU systems.

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New Images


┌─────────────────────────────────┬────────────────────────────────────────────────┬──────────────────────────────────────────────────┐
│ Image                           │ Description                                    │ Status                                           │
├─────────────────────────────────┼────────────────────────────────────────────────┼──────────────────────────────────────────────────┤
│ opencentauri-upgrade-hh-full    │ Standard COSMOS + Happy Hare + Mainsail/Fluidd │ ⚠️ Untested - likely exceeds 128MB RAM limit     │
├─────────────────────────────────┼────────────────────────────────────────────────┼──────────────────────────────────────────────────┤
│ opencentauri-upgrade-hh-minimal │ Happy Hare without web frontend                │ ✅ Recommended - optimized for limited resources │
└─────────────────────────────────┴────────────────────────────────────────────────┴──────────────────────────────────────────────────┘

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Key Features

Happy Hare Integration
 - Full MMU software suite with configuration files in /etc/klipper/config/mmu/
 - Kalico firmware for MMU MCU board (ERB, etc.)
 - MMU-specific macros and hardware configs adapted for OpenCentauri paths

HH-Minimal Design
 - Removes memory-heavy packages: Mainsail, Fluidd, atomscreen, guppyscreen, grumpyscreen, ustreamer, fbdoom
 - Moonraker API remains accessible on port 80
 - Use tools/local-mainsail/ to run Mainsail on external PC

Docker Build Environment
 - tools/local-build/ - Complete Yocto build setup in Docker
 - Bindgen cross-compilation fix for Rust packages (atomscreen)
 - Automatic site.conf cleanup after build

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Removed Packages (HH-Minimal only)

 - fluidd, mainsail - No browser interface on printer
 - atomscreen, guppyscreen, grumpyscreen - No touchscreen interface
 - ustreamer - No webcam streaming via moonraker
 - fbdoom - No DOOM game (no display available)
 - v4l-utils - No webcam configuration tools

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Network Setup Required

HH-Minimal has no touchscreen interface. Users must configure network access before flashing:

 - USB Ethernet Adapter - Auto-detected
 - WiFi via USB Stick - Place wpa_supplicant.conf on USB, auto-copied on insertion

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Documentation

 - README-happy-hare.md - Detailed HH image documentation
 - tools/local-build/README.md - Docker build instructions
 - tools/local-mainsail/README.md - Local Mainsail setup guide

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Important Notes

 - Happy Hare `install.sh` must be run externally to generate MMU-specific configs
 - Network connectivity should be tested on base COSMOS image before flashing HH-Minimal
 - Chamber lighting will not work (v4l-utils removed, no v4l2-ctl)

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> Note: Release publishing configuration was intentionally not included. This decision is left to project maintainers.