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

DESIGN.md

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+# Design Principles
+
+These are the non-negotiable first principles of this project. Every design decision,
+naming choice, module boundary, and abstraction is evaluated against them. When in
+doubt, choose simplicity and clarity over cleverness.
+
+## What This Is
+
+A transparent LoRa radio over USB. Plug the dongle into your PC, talk to any LoRa
+radio within range. That's it.
+
+## What This Is Not
+
+Not a mesh network node. Not a Meshtastic device. Not a protocol implementation. Not
+an application framework. This firmware has no opinion about what you do with your
+radio — it just gives you clean access to it.
+
+## Principles
+
+1. **Single purpose.** Transparent LoRa radio over USB. No mesh, no routing, no
+   application logic. Ever.
+
+2. **Protocol-agnostic.** The firmware is a dumb pipe. It exposes clean LoRa
+   parameters (frequency, bandwidth, spreading factor, coding rate, sync word, TX
+   power) and nothing else. No raw register access, no chip-specific leakage.
+
+3. **Host-driven.** The firmware makes zero autonomous decisions. The radio idles on
+   boot until the host commands it. No persistence across reboots. The host is always
+   in control.
+
+4. **Zero-config.** Boots, enumerates USB, waits for commands. The host plugs in and
+   goes.
+
+5. **Robustness over features.** Tasks never panic. Errors are reported to the host.
+   The radio self-recovers. Ship fewer features, but make them bulletproof.
+
+6. **Minimal footprint.** Avoid heavy dependencies. Keep the binary small. Every
+   dependency must justify its inclusion.
+
+7. **Auto-detect peripherals.** If the board has a display, use it. No user
+   configuration required.
+
+8. **World-class Rust.** Fully leverage the type system for safety. Strict `no_std`.
+   Always clippy-clean. Thoughtfully designed — everything has an obvious home.
+   Anyone who reads this code should immediately think *"this is nice."*
+
+9. **Easy board support.** Adding a new board means dropping a `.rs` file and adding
+   a Cargo feature. Nothing else changes.
+
+## Architecture
+
+```
+Host PC
+  │ USB CDC-ACM (COBS-framed postcard)
+  ▼
+usb_task ──Command──► radio_task ──► SX1262
+         ◄──Response──     │
+                      StatusWatch
+                           ▼
+                     display_task (optional) ──► OLED/TFT
+```
+
+Each task exclusively owns its peripheral. No shared state except typed channels and
+a watch signal. Pure actor model.
+
+## Technical Decisions
+
+| Decision | Choice | Why |
+|---|---|---|
+| USB framing | COBS + postcard | Compact, `no_std` native, battle-tested |
+| Serialization | postcard + serde | Zero-alloc binary codec, perfect for embedded |
+| Async runtime | Embassy | De facto standard for async embedded Rust |
+| Display | Radio status dashboard | Freq, BW, SF, packet counts, RSSI/SNR |
+| Boot behavior | Idle until host commands | Host-driven principle |
+| Persistence | None | Always start fresh, true dumb pipe |
+| Board codegen | Jinja2 via build.rs | Auto-discovers boards from filesystem |