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/basic-toml/tests/enum_external_deserialize.rs

@@ -0,0 +1,30 @@
+#![allow(clippy::wildcard_imports)]
+
+use serde::Deserialize;
+
+#[derive(Debug, Deserialize, PartialEq)]
+struct Struct {
+    value: Enum,
+}
+
+#[derive(Debug, Deserialize, PartialEq)]
+enum Enum {
+    Variant,
+}
+
+#[test]
+fn unknown_variant() {
+    let error = basic_toml::from_str::<Struct>("value = \"NonExistent\"").unwrap_err();
+
+    assert_eq!(
+        error.to_string(),
+        "unknown variant `NonExistent`, expected `Variant` for key `value` at line 1 column 1"
+    );
+}
+
+#[test]
+fn from_str() {
+    let s = basic_toml::from_str::<Struct>("value = \"Variant\"").unwrap();
+
+    assert_eq!(Enum::Variant, s.value);
+}