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/semver/tests/serde.rs

@@ -0,0 +1,90 @@
+#![cfg(feature = "serde")]
+
+#[macro_use]
+extern crate serde_derive;
+
+extern crate semver;
+extern crate serde_json;
+
+use semver::{Identifier, Version, VersionReq};
+
+#[derive(Serialize, Deserialize, PartialEq, Debug)]
+struct Identified {
+    name: String,
+    identifier: Identifier,
+}
+
+#[derive(Serialize, Deserialize, PartialEq, Debug)]
+struct Versioned {
+    name: String,
+    vers: Version,
+}
+
+#[test]
+fn serialize_identifier() {
+    let id = Identified {
+        name: "serde".to_owned(),
+        identifier: Identifier::Numeric(100),
+    };
+    let j = serde_json::to_string(&id).unwrap();
+    assert_eq!(j, r#"{"name":"serde","identifier":100}"#);
+
+    let id = Identified {
+        name: "serde".to_owned(),
+        identifier: Identifier::AlphaNumeric("b100".to_owned()),
+    };
+    let j = serde_json::to_string(&id).unwrap();
+    assert_eq!(j, r#"{"name":"serde","identifier":"b100"}"#);
+}
+
+#[test]
+fn deserialize_identifier() {
+    let j = r#"{"name":"serde","identifier":100}"#;
+    let id = serde_json::from_str::<Identified>(j).unwrap();
+    let expected = Identified {
+        name: "serde".to_owned(),
+        identifier: Identifier::Numeric(100),
+    };
+    assert_eq!(id, expected);
+
+    let j = r#"{"name":"serde","identifier":"b100"}"#;
+    let id = serde_json::from_str::<Identified>(j).unwrap();
+    let expected = Identified {
+        name: "serde".to_owned(),
+        identifier: Identifier::AlphaNumeric("b100".to_owned()),
+    };
+    assert_eq!(id, expected);
+}
+
+#[test]
+fn serialize_version() {
+    let v = Versioned {
+        name: "serde".to_owned(),
+        vers: Version::parse("1.0.0").unwrap(),
+    };
+    let j = serde_json::to_string(&v).unwrap();
+    assert_eq!(j, r#"{"name":"serde","vers":"1.0.0"}"#);
+}
+
+#[test]
+fn deserialize_version() {
+    let j = r#"{"name":"serde","vers":"1.0.0"}"#;
+    let v = serde_json::from_str::<Versioned>(j).unwrap();
+    let expected = Versioned {
+        name: "serde".to_owned(),
+        vers: Version::parse("1.0.0").unwrap(),
+    };
+    assert_eq!(v, expected);
+}
+
+#[test]
+fn serialize_versionreq() {
+    let v = VersionReq::exact(&Version::parse("1.0.0").unwrap());
+
+    assert_eq!(serde_json::to_string(&v).unwrap(), r#""= 1.0.0""#);
+}
+
+#[test]
+fn deserialize_versionreq() {
+    assert_eq!("1.0.0".parse::<VersionReq>().unwrap(), serde_json::from_str(r#""1.0.0""#).unwrap());
+}