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

Cargo.toml

@@ -0,0 +1,49 @@
+[package]
+name = "lora-dongle"
+version = "0.1.0"
+edition = "2021"
+build = "build.rs"
+
+[dependencies]
+cfg-if = "1.0"
+critical-section = "1.2"
+defmt = "0.3"
+embassy-embedded-hal = "0.2"
+embassy-executor = { version = "0.6", features = ["task-arena-size-12288", "executor-thread"] }
+embassy-sync = "0.6"
+embassy-time = "0.3"
+embassy-usb = { version = "0.3", features = ["defmt"] }
+heapless = { version = "0.8", features = ["serde"] }
+lora-phy = "3.0"
+postcard = { version = "1.0", default-features = false }
+serde = { version = "1.0", default-features = false, features = ["derive"] }
+static_cell = "2.1"
+
+# ESP32-S3 (Heltec V3)
+esp-backtrace = { version = "0.14", optional = true, features = ["esp32s3", "panic-handler", "defmt"] }
+esp-hal = { version = "0.22", optional = true, features = ["esp32s3"] }
+esp-hal-embassy = { version = "0.5", optional = true, features = ["esp32s3"] }
+esp-println = { version = "0.12", optional = true, features = ["esp32s3", "defmt-espflash"] }
+
+# nRF52840 (RAK 4631)
+cortex-m-rt = { version = "0.7", optional = true }
+embassy-nrf = { version = "0.2", optional = true, features = ["nrf52840", "gpiote", "time-driver-rtc1", "defmt"] }
+panic-probe = { version = "0.3", optional = true, features = ["print-defmt"] }
+
+[build-dependencies]
+minijinja = "2.0"
+owo-colors = "4.0"
+
+[features]
+heltec_v3 = [
+    "dep:esp-backtrace",
+    "dep:esp-hal",
+    "dep:esp-hal-embassy",
+    "dep:esp-println",
+]
+rak_4631 = [
+    "dep:cortex-m-rt",
+    "dep:embassy-nrf",
+    "dep:panic-probe",
+    "embassy-executor/arch-cortex-m",
+]