feat: initial lora-dongle firmware scaffold

Transparent LoRa-over-USB dongle firmware supporting Heltec V3 (ESP32-S3)
and RAK 4631 (nRF52840) boards. Actor-model architecture with typed
channels between USB, radio, and display tasks.

- Protocol: Command/Response enums with COBS+postcard framing design
- Board abstraction: auto-discovered via build.rs Jinja2 codegen
- Radio task: SX1262 state machine (Idle → Receiving ↔ Transmitting)
- Display task: optional SSD1306 dashboard via StatusWatch
- RAK 4631: fully wired SPI/USB/I2C peripherals, clippy-clean
- Heltec V3: structure in place, awaits espup Xtensa toolchain

src/board/mod.rs.j2

@@ -0,0 +1,9 @@
+// @generated by build.rs — DO NOT EDIT
+cfg_if::cfg_if! {
+    {%- for board in boards %}
+    {% if loop.first %}if{% else %}else if{% endif %} #[cfg(feature = "{{ board }}")] {
+        mod {{ board }};
+        pub use {{ board }}::*;
+    }
+    {%- endfor %}
+}