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)

build.rs

@@ -8,7 +8,8 @@ fn main() {
         .expect("failed to read src/board directory")
         .filter_map(|e| {
             let p = e.ok()?.path();
-            if p.is_file() && p.extension()? == "rs" && p.file_stem()? != "mod" {
+            let stem = p.file_stem()?.to_str()?;
+            if p.is_file() && p.extension()? == "rs" && stem != "mod" && stem != "traits" {
                 return Some(p.file_stem()?.to_str()?.to_string());
             }
             None