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).

Justfile

@@ -1,10 +1,8 @@
 set shell := ["bash", "-c"]
-heltec_v3 := "heltec_v3 xtensa-esp32s3-none-elf esp32s3"
-rak_4631  := "rak_4631 thumbv7em-none-eabihf nRF52840_xxAA"
+rak_4631 := "rak_4631 thumbv7em-none-eabihf nRF52840_xxAA"
 
 # Check all boards compile cleanly
 check-all:
-    just check heltec_v3
     just check rak_4631
 
 # Check a single board compiles
@@ -30,6 +28,5 @@ size board:
 
 [private]
 _info name:
-    @if [ "{{name}}" == "heltec_v3" ]; then echo "{{heltec_v3}}"; \
-     elif [ "{{name}}" == "rak_4631" ]; then echo "{{rak_4631}}"; \
-     else echo "Unknown board" >&2; exit 1; fi
+    @if [ "{{name}}" == "rak_4631" ]; then echo "{{rak_4631}}"; \
+     else echo "Unknown board: {{name}}" >&2; exit 1; fi