Fix K8850 key binding protocol (use native FD command)

The 8850 firmware uses its own 0xFD command for key bindings, not 0xFE
like the K884x family. This replaces the K884x delegation with a native
implementation that correctly handles:

- Single keys (binding_mode=1, compact format)
- Modifier combos using firmware IDs F1-F4 (not HID bitmasks)
- Multi-key macros using unified triplet format [action, 0x00, 0x32]
- Media keys and mouse actions
- Finalize packet [03 FD FE FF] to commit config

Also fixes open_device to send the correct 0xFB init for 8850 instead
of 64 zeros which corrupts the device config, and fixes set_led to send
all 3 layers as firmware requires.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: skarard