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/protocol.rs

@@ -4,10 +4,6 @@ use serde::{Deserialize, Serialize};
 /// Maximum LoRa payload size in bytes.
 pub const MAX_PAYLOAD: usize = 256;
 
-/// Protocol version for host compatibility checks.
-#[allow(dead_code)]
-pub const PROTOCOL_VERSION: u8 = 1;
-
 /// LoRa signal bandwidth.
 #[derive(Serialize, Deserialize, Debug, Clone, Copy, PartialEq, Eq, defmt::Format)]
 #[repr(u8)]