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/darling/tests/compiletests.rs

@@ -0,0 +1,16 @@
+#![cfg(compiletests)]
+
+#[rustversion::stable(1.77)]
+#[test]
+fn compile_test() {
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/compile-fail/*.rs");
+}
+
+#[rustversion::not(stable(1.77))]
+#[test]
+fn wrong_rustc_version() {
+    panic!(
+        "This is not the expected version of rustc. Error messages vary across compiler versions so tests may produce spurious errors"
+    );
+}