feat: initial qmk-host Raw HID bridge implementation

Add a Rust daemon that connects to QMK keyboards over their Raw HID
interface (VID 0x3434, usage page 0xFF60 / usage 0x0061) and provides:

- Bi-directional layer sync: CMD_LAYER_SYNC (0x40) packets are bridged
  between all connected keyboards so their active layers stay in sync.
  On connect, a FLAG_QUERY packet requests the current state from each
  keyboard. A second keyboard (numpad) is stubbed and ready to enable.

- Volume monitoring: polls `pactl get-sink-volume @DEFAULT_SINK@` every
  2 s and pushes CMD_VOLUME (0x41) packets on change. Compatible with
  both PulseAudio and PipeWire on KDE6/Wayland.

- Brightness monitoring: polls /sys/class/backlight on the same interval
  and pushes CMD_BRIGHTNESS (0x42) packets on change.

- Active-app stub: CMD_ACTIVE_APP (0x43) is reserved with detailed notes
  on KDE6/Wayland implementation approaches (KWin D-Bus, foreign
  toplevel protocol) for a future commit.

Each keyboard runs a single combined read/write IO thread using a 10 ms
read timeout to interleave HID reads and queued writes without needing
two file handles. Protocol constants in src/protocol.rs mirror
hid_protocol.h in the QMK firmware exactly.

Licensed under GPL-3.0-or-later.

Cargo.toml

@@ -0,0 +1,17 @@
+[package]
+name = "qmk-host"
+version = "0.1.0"
+edition = "2021"
+description = "Host bridge for bi-directional Raw HID layer sync and host data delivery to QMK keyboards"
+license = "GPL-3.0-or-later"
+
+[[bin]]
+name = "qmk-host"
+path = "src/main.rs"
+
+[dependencies]
+hidapi   = "2"          # Cross-platform Raw HID access
+anyhow   = "1"          # Ergonomic error handling
+log      = "0.4"        # Logging facade
+env_logger = "0.11"     # Logger implementation (RUST_LOG env var)
+ctrlc    = "3"          # Ctrl+C / SIGINT handler