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/pulldown-cmark/tests/serde.rs

@@ -0,0 +1,86 @@
+#[cfg(feature = "serde")]
+mod tests {
+    use std::convert::TryInto;
+
+    use pulldown_cmark::CowStr;
+
+    #[test]
+    fn escaped() {
+        let must_be_escaped = "\"";
+        let wire = serde_json::to_string(must_be_escaped).unwrap();
+        let de = serde_json::from_str::<CowStr>(&wire).unwrap();
+        assert_eq!(&*de, must_be_escaped);
+    }
+
+    #[test]
+    fn cow_str_to_str_round_trip_bincode() {
+        for i in &[
+            CowStr::Borrowed("dssa"),
+            CowStr::Borrowed(""),
+            CowStr::Boxed("hello".to_owned().into_boxed_str()),
+            CowStr::Boxed("".to_owned().into_boxed_str()),
+            CowStr::Inlined("inline".try_into().unwrap()),
+            CowStr::Inlined("".try_into().unwrap()),
+        ] {
+            let encoded = bincode::serialize(i).unwrap();
+            let decoded1: CowStr = bincode::deserialize(&encoded).unwrap();
+            let decoded2: String = bincode::deserialize(&encoded).unwrap();
+            let decoded3: &str = bincode::deserialize(&encoded).unwrap();
+
+            assert_eq!(&decoded1, i);
+            assert_eq!(decoded2, i.as_ref());
+            assert_eq!(decoded3, i.as_ref());
+        }
+    }
+
+    #[test]
+    fn cow_str_to_str_round_trip_json() {
+        for i in &[
+            CowStr::Borrowed("dssa"),
+            CowStr::Borrowed(""),
+            CowStr::Boxed("hello".to_owned().into_boxed_str()),
+            CowStr::Boxed("".to_owned().into_boxed_str()),
+            CowStr::Inlined("inline".try_into().unwrap()),
+            CowStr::Inlined("".try_into().unwrap()),
+        ] {
+            let encoded = serde_json::to_string(i).unwrap();
+            let decoded1: CowStr = serde_json::from_str(&encoded).unwrap();
+            let decoded2: String = serde_json::from_str(&encoded).unwrap();
+            let decoded3: &str = serde_json::from_str(&encoded).unwrap();
+
+            assert_eq!(&decoded1, i);
+            assert_eq!(decoded2, i.as_ref());
+            assert_eq!(decoded3, i.as_ref());
+        }
+    }
+
+    #[test]
+    fn str_to_cow_str_json() {
+        let str = "a borrowed str";
+        let string = "a owned str".to_owned();
+
+        let encoded_str = serde_json::to_string(&str).unwrap();
+        let encoded_string = serde_json::to_string(&string).unwrap();
+
+        let decoded_str: CowStr = serde_json::from_str(&encoded_str).unwrap();
+        let decoded_string: CowStr = serde_json::from_str(&encoded_string).unwrap();
+
+        assert_eq!(decoded_str.as_ref(), str);
+        assert_eq!(decoded_string.as_ref(), string);
+    }
+
+    #[test]
+    fn str_to_cow_str_bincode() {
+        let str = "a borrowed str";
+        let string = "a owned str".to_owned();
+
+        let encoded_str = bincode::serialize(&str).unwrap();
+        let encoded_string = bincode::serialize(&string).unwrap();
+
+        let decoded_str: CowStr = bincode::deserialize(&encoded_str).unwrap();
+        let decoded_string: CowStr = bincode::deserialize(&encoded_string).unwrap();
+
+        assert_eq!(decoded_str.as_ref(), str);
+        assert_eq!(decoded_string.as_ref(), string);
+    }
+}