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/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 _;
     }
 }