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/float.rs

@@ -0,0 +1,81 @@
+#![allow(clippy::float_cmp)]
+
+use serde::{Deserialize, Serialize};
+use serde_json::Value;
+
+#[rustfmt::skip] // appears to be a bug in rustfmt to make this converge...
+macro_rules! float_inf_tests {
+    ($ty:ty) => {{
+        #[derive(Serialize, Deserialize)]
+        struct S {
+            sf1: $ty,
+            sf2: $ty,
+            sf3: $ty,
+            sf4: $ty,
+            sf5: $ty,
+            sf6: $ty,
+            sf7: $ty,
+            sf8: $ty,
+        }
+        let inf: S = basic_toml::from_str(
+            "
+        # infinity
+        sf1 = inf  # positive infinity
+        sf2 = +inf # positive infinity
+        sf3 = -inf # negative infinity
+
+        # not a number
+        sf4 = nan  # actual sNaN/qNaN encoding is implementation specific
+        sf5 = +nan # same as `nan`
+        sf6 = -nan # valid, actual encoding is implementation specific
+
+        # zero
+        sf7 = +0.0
+        sf8 = -0.0
+        ",
+        )
+        .expect("Parse infinities.");
+
+        assert!(inf.sf1.is_infinite());
+        assert!(inf.sf1.is_sign_positive());
+        assert!(inf.sf2.is_infinite());
+        assert!(inf.sf2.is_sign_positive());
+        assert!(inf.sf3.is_infinite());
+        assert!(inf.sf3.is_sign_negative());
+
+        assert!(inf.sf4.is_nan());
+        assert!(inf.sf4.is_sign_positive());
+        assert!(inf.sf5.is_nan());
+        assert!(inf.sf5.is_sign_positive());
+        assert!(inf.sf6.is_nan());
+        assert!(inf.sf6.is_sign_negative()); // NOTE: but serializes to just `nan`
+
+        assert_eq!(inf.sf7, 0.0);
+        assert!(inf.sf7.is_sign_positive());
+        assert_eq!(inf.sf8, 0.0);
+        assert!(inf.sf8.is_sign_negative());
+
+        let s = basic_toml::to_string(&inf).unwrap();
+        assert_eq!(
+            s,
+            "\
+sf1 = inf
+sf2 = inf
+sf3 = -inf
+sf4 = nan
+sf5 = nan
+sf6 = nan
+sf7 = 0.0
+sf8 = -0.0
+"
+        );
+
+        basic_toml::from_str::<Value>(&s).expect("roundtrip");
+    }};
+}
+
+#[test]
+fn float_inf() {
+    float_inf_tests!(f32);
+    float_inf_tests!(f64);
+}