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

build.rs

@@ -0,0 +1,35 @@
+use minijinja::{context, Environment};
+use owo_colors::OwoColorize;
+use std::fs;
+
+fn main() {
+    let board_dir = "src/board";
+    let boards: Vec<String> = fs::read_dir(board_dir).unwrap()
+        .filter_map(|e| {
+            let p = e.ok()?.path();
+            if p.is_file() && p.extension()? == "rs" && p.file_stem()? != "mod" {
+                return Some(p.file_stem()?.to_str()?.to_string());
+            }
+            None
+        }).collect();
+
+    let enabled: Vec<_> = std::env::vars()
+        .filter(|(k, _)| k.starts_with("CARGO_FEATURE_"))
+        .map(|(k, _)| k.replace("CARGO_FEATURE_", "").to_lowercase())
+        .filter(|f| boards.contains(f)).collect();
+
+    match enabled.len() {
+        0 => panic!("\n\n{}\n", format!("No board selected! Found: {:?}", boards).red().bold()),
+        1 => println!("cargo:rustc-cfg=board_selected"),
+        _ => panic!("\n\n{}\n", format!("Conflict: Multiple boards enabled {:?}", enabled).yellow().bold()),
+    }
+
+    let mut env = Environment::new();
+    let template = fs::read_to_string("src/board/mod.rs.j2").expect("Missing template .j2");
+    env.add_template("mod", &template).unwrap();
+    let rendered = env.get_template("mod").unwrap().render(context!(boards => boards)).unwrap();
+    fs::write("src/board/mod.rs", rendered).unwrap();
+
+    println!("cargo:rerun-if-changed=src/board/mod.rs.j2");
+    println!("cargo:rerun-if-changed={}", board_dir);
+}