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/benches/common.rs

@@ -0,0 +1,58 @@
+use embedded_graphics::prelude::*;
+use std::convert::TryFrom;
+
+const SIZE: usize = 256;
+
+pub struct Framebuffer<C> {
+    pixels: [[C; SIZE]; SIZE],
+}
+
+impl<C> Framebuffer<C>
+where
+    C: PixelColor + Default,
+{
+    pub fn new() -> Self {
+        Self {
+            pixels: [[C::default(); 256]; 256],
+        }
+    }
+
+    fn set_pixel(&mut self, position: Point, color: C) {
+        if let (Ok(x), Ok(y)) = (usize::try_from(position.x), usize::try_from(position.y)) {
+            self.pixels
+                .get_mut(y)
+                .and_then(|row| row.get_mut(x))
+                .map(|pixel| *pixel = color);
+        } else {
+            panic!(
+                "tried to set pixel outside the framebuffer at {:?}",
+                position
+            );
+        }
+    }
+}
+
+impl<C> DrawTarget for Framebuffer<C>
+where
+    C: PixelColor + Default,
+{
+    type Color = C;
+    type Error = core::convert::Infallible;
+
+    fn draw_iter<I>(&mut self, pixels: I) -> Result<(), Self::Error>
+    where
+        I: IntoIterator<Item = Pixel<Self::Color>>,
+    {
+        for Pixel(position, color) in pixels {
+            self.set_pixel(position, color);
+        }
+
+        Ok(())
+    }
+}
+
+impl<C> OriginDimensions for Framebuffer<C> {
+    fn size(&self) -> Size {
+        Size::new_equal(SIZE as u32)
+    }
+}