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/display/render.rs

@@ -1,4 +1,4 @@
-use crate::channel::{RadioState, RadioStatus};
+use crate::channel::RadioStatus;
 
 /// Render the status dashboard to a framebuffer.
 ///
@@ -13,13 +13,3 @@ pub fn dashboard(status: &RadioStatus) {
     //   Line 5: last RSSI / SNR
     let _ = status;
 }
-
-/// Format radio state as a short display string.
-#[allow(dead_code)]
-pub fn state_label(state: RadioState) -> &'static str {
-    match state {
-        RadioState::Idle => "IDLE",
-        RadioState::Receiving => "RX",
-        RadioState::Transmitting => "TX",
-    }
-}