feat: restore Heltec V3, add firmware output, support all common boards

Reverts the removal of heltec_v3 — it's a major board. Xtensa builds
use the esp toolchain (+esp via espup); check-all gracefully skips
boards whose toolchain isn't installed.

Build artifacts now land in firmware/ with readable names like
lora-dongle-rak_4631-release.elf instead of being buried in target/.

DESIGN.md updated with "support all common boards" principle (#10)
and corrected toolchain policy (#11).

src/board/heltec_v3.rs

@@ -0,0 +1,48 @@
+use embassy_time::Delay;
+
+use lora_phy::iv::GenericSx126xInterfaceVariant;
+use lora_phy::sx126x::{Sx1262, Sx126x};
+
+// ── Concrete peripheral types ────────────────────────────────────────
+
+// TODO: replace these placeholders with real esp-hal peripheral types
+//   SPI2 for SX1262 (SCK=9, MOSI=10, MISO=11, NSS=8)
+//   GPIO for radio control (RST=12, DIO1=14, BUSY=13)
+//   Vext pin (GPIO21) LOW to power OLED
+//   I2C0 for SSD1306 (SDA=17, SCL=18)
+//   USB OTG for CDC-ACM
+
+type Iv = GenericSx126xInterfaceVariant<(), ()>;
+type RadioSpiDevice = ();
+pub type RadioDriver = Sx126x<RadioSpiDevice, Iv, Sx1262>;
+
+// ── Peripheral bundles ───────────────────────────────────────────────
+
+pub struct RadioParts {
+    pub driver: RadioDriver,
+    pub delay: Delay,
+}
+
+pub struct UsbParts {
+    pub driver: (),
+}
+
+pub struct DisplayParts {
+    pub i2c: (),
+}
+
+// ── Board init ───────────────────────────────────────────────────────
+
+pub struct Board;
+
+impl Board {
+    pub fn init() -> Self {
+        // TODO: call esp_hal::init(esp_hal::Config::default())
+        Self
+    }
+
+    pub fn into_parts(self) -> (RadioParts, UsbParts, Option<DisplayParts>) {
+        // TODO: wire up real peripherals once building with nightly + -Zbuild-std
+        todo!("Heltec V3 peripheral init")
+    }
+}

src/main.rs

@@ -15,6 +15,8 @@ use crate::channel::{CommandChannel, ResponseChannel, StatusWatch};
 cfg_if::cfg_if! {
     if #[cfg(feature = "rak_4631")] {
         use panic_probe as _;
+    } else if #[cfg(feature = "heltec_v3")] {
+        use esp_backtrace as _;
     }
 }