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/embedded-graphics/tests/chaining.rs

@@ -0,0 +1,76 @@
+extern crate embedded_graphics;
+
+use embedded_graphics::{
+    prelude::*,
+    primitives::{Circle, Line, Primitive, PrimitiveStyle, Rectangle},
+};
+
+struct FakeDisplay {}
+
+#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+pub struct TestPixelColor(pub bool);
+
+impl PixelColor for TestPixelColor {
+    type Raw = ();
+}
+
+impl From<u8> for TestPixelColor {
+    fn from(other: u8) -> Self {
+        TestPixelColor(other != 0)
+    }
+}
+
+impl DrawTarget for FakeDisplay {
+    type Color = TestPixelColor;
+    type Error = core::convert::Infallible;
+
+    fn draw_iter<I>(&mut self, _pixels: I) -> Result<(), Self::Error>
+    where
+        I: IntoIterator<Item = Pixel<Self::Color>>,
+    {
+        Ok(())
+    }
+}
+
+impl OriginDimensions for FakeDisplay {
+    fn size(&self) -> Size {
+        Size::zero()
+    }
+}
+
+#[test]
+fn it_supports_chaining() -> Result<(), core::convert::Infallible> {
+    let mut display = FakeDisplay {};
+
+    let chained = Rectangle::new(Point::new(0, 0), Size::new(1, 1))
+        .into_styled(PrimitiveStyle::default())
+        .pixels()
+        .chain(
+            Circle::new(Point::new(1, 1), 3)
+                .into_styled(PrimitiveStyle::default())
+                .pixels(),
+        );
+
+    chained.draw(&mut display)
+}
+
+fn multi() -> impl Iterator<Item = Pixel<TestPixelColor>> {
+    let line = Line::new(Point::new(0, 1), Point::new(2, 3))
+        .into_styled(PrimitiveStyle::with_stroke(1u8.into(), 1))
+        .pixels();
+
+    let circle = Circle::new(Point::new(2, 2), 7)
+        .into_styled(PrimitiveStyle::with_stroke(1u8.into(), 1))
+        .pixels();
+
+    line.chain(circle)
+}
+
+#[test]
+fn return_from_fn() -> Result<(), core::convert::Infallible> {
+    let mut display = FakeDisplay {};
+
+    let chained = multi();
+
+    chained.draw(&mut display)
+}