feat: Firmware update via USB (DFU mode)

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Feature Description

Enable firmware updates without physically pressing BOOTSEL button.

Motivation

• Easier firmware updates
• Remote updates possible
• Better user experience
• Professional feature

Implementation Options

Option A: USB DFU (Device Firmware Update)

• Standard USB DFU protocol
• Use dfu-util on Linux
• Supported by TinyUSB
• Industry standard

Option B: Custom Bootloader

• Custom USB protocol
• Smaller footprint
• More control over process

Option C: UF2 Upload via HID

• Upload UF2 file via HID reports
• Write to flash
• Reboot automatically

Usage

Linux

# Enter DFU mode
pico-fan-ctl firmware update-mode

# Flash new firmware
dfu-util -a 0 -D pico_fan_hub_v1.1.0.uf2

# Or automated:
pico-fan-ctl firmware flash pico_fan_hub_v1.1.0.uf2

Safety Features

• Checksum verification
• Dual-partition fallback
• Bootloader protection
• Recovery mode
• Never brick the device

Implementation Notes

• Partition flash: Bootloader + App
• Reserve ~16KB for bootloader
• Use RP2040 reset to bootloader
• TinyUSB DFU class

Acceptance Criteria

• Enter DFU mode via USB command
• Update firmware without BOOTSEL button
• Checksum verification
• Safe rollback on failure
• CLI tool support
• Documentation

Priority

Low - Convenience feature