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/display-tricky.rs

@@ -0,0 +1,53 @@
+use serde::{Deserialize, Serialize};
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Recipe {
+    pub name: String,
+    pub description: Option<String>,
+    #[serde(default)]
+    pub modules: Vec<Modules>,
+    #[serde(default)]
+    pub packages: Vec<Packages>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Modules {
+    pub name: String,
+    pub version: Option<String>,
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct Packages {
+    pub name: String,
+    pub version: Option<String>,
+}
+
+#[test]
+fn both_ends() {
+    let recipe_works = basic_toml::from_str::<Recipe>(
+        r#"
+        name = "testing"
+        description = "example"
+        modules = []
+
+        [[packages]]
+        name = "base"
+    "#,
+    )
+    .unwrap();
+    basic_toml::to_string(&recipe_works).unwrap();
+
+    let recipe_fails = basic_toml::from_str::<Recipe>(
+        r#"
+        name = "testing"
+        description = "example"
+        packages = []
+
+        [[modules]]
+        name = "base"
+    "#,
+    )
+    .unwrap();
+    let err = basic_toml::to_string(&recipe_fails).unwrap_err();
+    assert_eq!(err.to_string(), "values must be emitted before tables");
+}