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/defmt/tests/derive-bounds.rs

@@ -0,0 +1,36 @@
+extern crate defmt as defmt2;
+
+fn main() {
+    let baz: Baz<Qux> = Default::default();
+    defmt::info!("{}", baz);
+}
+
+trait Foo {
+    type Bar;
+}
+#[derive(defmt::Format, Default)]
+struct Baz<T: Foo> {
+    field: T::Bar,
+    field2: Quux<T>,
+}
+#[derive(defmt::Format, Default)]
+struct Qux;
+impl Foo for Qux {
+    type Bar = Qux;
+}
+#[allow(dead_code)]
+#[derive(defmt::Format, Default)]
+enum Quux<T: Foo> {
+    #[default]
+    None,
+    Variant1(T),
+    Variant2 {
+        f: T::Bar,
+    },
+    Variant3(T::Bar),
+}
+
+#[allow(dead_code)]
+#[derive(defmt::Format)]
+#[defmt(crate = defmt2)]
+struct Quz;