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)

Cargo.toml

@@ -18,6 +18,7 @@ categories = ["embedded", "hardware-support", "no-std"]
 cfg-if = "1.0"
 critical-section = "1.2"
 defmt = "0.3"
+embedded-hal-async = "1.0"
 embassy-embedded-hal = "0.2"
 embassy-executor = { version = "0.6", features = ["task-arena-size-32768"] }
 embassy-sync = "0.6"