feat: Multiple RGB channels and zones

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Feature Description

Support multiple independent RGB channels/zones for complex setups.

Motivation

• Different case zones need different colors
• Multiple RGB strips
• Front vs. back lighting
• Per-fan RGB (if fans have integrated LEDs)

Implementation

Hardware

• Additional GPIO pins for RGB data
• Up to 4-6 independent channels
• Each channel: separate WS2812B chain

Pin Usage

• Channel 1: GPIO 16 (existing)
• Channel 2: GPIO 17
• Channel 3: GPIO 18
• Channel 4: GPIO 19
• etc.

Software Control

Per-Zone:

pico-fan-ctl rgb zone 1 rainbow
pico-fan-ctl rgb zone 2 static --color ff0000
pico-fan-ctl rgb zone 3 breathing --color 0000ff

Synchronized:

pico-fan-ctl rgb all rainbow --sync

USB HID Extension

• Report 0x03 extended with zone parameter
• Configure LEDs per zone
• Zone mapping stored in flash

Use Cases

1. Case Lighting: Front + back different colors
2. Fan RGB: Individual fan ring colors
3. Strip Zones: Top/bottom/side strips
4. Ambient: Case + desk lighting
5. Status: Different zones for different info

Acceptance Criteria

• At least 3 RGB channels
• Independent control
• Synchronization option
• Zone configuration in flash
• Documentation with wiring

Priority

Low - Advanced feature