refactor: LoRaBoard trait for compile-time board completeness verification

Every board's Board struct now implements the LoRaBoard trait, which
enforces that NAME, TX_POWER_RANGE, init(), and mac_address() are all
provided. Missing any of these is a compile error — no "good intentions"
needed.

The trait verifies metadata and initialization. Tasks use the board's
concrete types directly (Embassy tasks must be concrete). Board-specific
type aliases (RadioDriver, UsbDriver, DisplayI2c) and struct definitions
(RadioParts, UsbParts, DisplayParts) remain in each board file.

- New: src/board/traits.rs (LoRaBoard trait definition)
- Modified: all 3 board files (implement trait, remove standalone constants)
- Modified: main.rs (import trait for init)
- Modified: radio/task.rs (use Board::TX_POWER_RANGE via trait)
- Modified: display/task.rs (use Board::NAME via trait)
- Modified: build.rs (exclude traits.rs from board discovery)
- Removed: embedded-hal-async direct dep (not needed for trait approach)

src/main.rs

@@ -24,6 +24,7 @@ mod usb;
 
 use embassy_executor::Spawner;
 
+use crate::board::LoRaBoard;
 use crate::channel::{CommandChannel, DisplayCommandChannel, ResponseChannel, StatusWatch};
 
 cfg_if::cfg_if! {
@@ -55,7 +56,7 @@ cfg_if::cfg_if! {
 }
 
 async fn run(spawner: Spawner) {
-    let board = board::Board::init();
+    let board = <board::Board as LoRaBoard>::init();
     let (radio, usb, display) = board.into_parts();
 
     let has_display = display.is_some();