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

Justfile

@@ -0,0 +1,35 @@
+set shell := ["bash", "-c"]
+heltec_v3 := "heltec_v3 xtensa-esp32s3-none-elf esp32s3"
+rak_4631  := "rak_4631 thumbv7em-none-eabihf nRF52840_xxAA"
+
+# Check all boards compile cleanly
+check-all:
+    just check heltec_v3
+    just check rak_4631
+
+# Check a single board compiles
+check board:
+    @read -r feat target chip <<< "$(just _info {{board}})"; \
+    cargo check --target $target --features $feat
+
+# Run clippy on a single board
+clippy board:
+    @read -r feat target chip <<< "$(just _info {{board}})"; \
+    cargo clippy --target $target --features $feat -- -D warnings
+
+# Build and flash a board via probe-rs
+flash board:
+    @read -r feat target chip <<< "$(just _info {{board}})"; \
+    cargo build --release --target $target --features $feat; \
+    probe-rs run --chip $chip --target $target --features $feat
+
+# Show binary size for a release build
+size board:
+    @read -r feat target chip <<< "$(just _info {{board}})"; \
+    cargo size --release --target $target --features $feat
+
+[private]
+_info name:
+    @if [ "{{name}}" == "heltec_v3" ]; then echo "{{heltec_v3}}"; \
+     elif [ "{{name}}" == "rak_4631" ]; then echo "{{rak_4631}}"; \
+     else echo "Unknown board" >&2; exit 1; fi