refactor: drop Heltec V3, enforce standard toolchain, remove dead code

Heltec V3 (ESP32-S3) requires the espup Xtensa toolchain fork, which
violates the standard-toolchain-only principle. Removed until upstream
Rust supports Xtensa targets via stock rustup.

Also removes all #[allow(dead_code)] band-aids — unused items are
deleted rather than suppressed. DESIGN.md updated with standard
toolchain principle (#10) and peripheral model (input/telemetry slots).

src/main.rs

@@ -4,7 +4,6 @@
 mod board;
 mod channel;
 mod display;
-mod error;
 mod protocol;
 mod radio;
 mod usb;
@@ -16,8 +15,6 @@ 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 _;
     }
 }