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).

Cargo.toml

@@ -19,12 +19,6 @@ postcard = { version = "1.0", default-features = false }
 serde = { version = "1.0", default-features = false, features = ["derive"] }
 static_cell = "2.1"
 
-# ESP32-S3 (Heltec V3)
-esp-backtrace = { version = "0.14", optional = true, features = ["esp32s3", "panic-handler", "defmt"] }
-esp-hal = { version = "0.22", optional = true, features = ["esp32s3"] }
-esp-hal-embassy = { version = "0.5", optional = true, features = ["esp32s3"] }
-esp-println = { version = "0.12", optional = true, features = ["esp32s3", "defmt-espflash"] }
-
 # nRF52840 (RAK 4631)
 cortex-m-rt = { version = "0.7", optional = true }
 embassy-nrf = { version = "0.2", optional = true, features = ["nrf52840", "gpiote", "time-driver-rtc1", "defmt"] }
@@ -35,12 +29,6 @@ minijinja = "2.0"
 owo-colors = "4.0"
 
 [features]
-heltec_v3 = [
-    "dep:esp-backtrace",
-    "dep:esp-hal",
-    "dep:esp-hal-embassy",
-    "dep:esp-println",
-]
 rak_4631 = [
     "dep:cortex-m-rt",
     "dep:embassy-nrf",