feat: add display UI with splash screen, dashboard, and RSSI sparkline

Implement full SSD1306 OLED display support:
- Splash screen on boot (1.5s) with project name, version, board
- Dashboard showing radio state, config, counters, RSSI/SNR
- RSSI history sparkline (64-sample ring buffer, 2px-wide bars)
- DisplayOn/DisplayOff host commands routed via dedicated channel
- Pure render functions against generic DrawTarget (no hardware coupling)
- Display on/off control bypasses radio task entirely

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/mutex-trait/ci/script.sh

@@ -0,0 +1,29 @@
+set -euxo pipefail
+
+main() {
+    if [ $TRAVIS_RUST_VERSION = nightly ]; then
+        export RUSTFLAGS="-D warnings"
+    fi
+
+    cargo check --target $TARGET
+
+    case $TARGET in
+        thumbv*-none-eabi*)
+            ;;
+
+        x86_64-unknown-linux-gnu)
+            cargo test --target $TARGET
+            ;;
+    esac
+
+    if [ $TRAVIS_RUST_VERSION = nightly ]; then
+        # Get the latest nightly with a working clippy
+        rustup toolchain uninstall nightly
+        rustup set profile default
+        rustup default nightly
+        rustup target add $TARGET
+        cargo clippy --target $TARGET -- -D warnings
+    fi
+}
+
+main