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/primitives_fixed_point.rs

@@ -0,0 +1,39 @@
+use criterion::*;
+use embedded_graphics::{geometry::AngleUnit, pixelcolor::Gray8, prelude::*, primitives::*};
+
+mod common;
+
+use common::Framebuffer;
+
+fn arc(c: &mut Criterion) {
+    c.bench_function("arc", |b| {
+        let object = &Arc::new(Point::new(100, 100), 100, -30.0.deg(), 150.0.deg())
+            .into_styled(PrimitiveStyle::with_stroke(Gray8::new(1), 1));
+
+        let mut framebuffer = Framebuffer::new();
+        b.iter(|| object.draw(&mut framebuffer))
+    });
+}
+
+fn sector(c: &mut Criterion) {
+    c.bench_function("sector", |b| {
+        let object = &Sector::new(Point::new(100, 100), 100, -30.0.deg(), 150.0.deg())
+            .into_styled(PrimitiveStyle::with_stroke(Gray8::new(1), 1));
+
+        let mut framebuffer = Framebuffer::new();
+        b.iter(|| object.draw(&mut framebuffer))
+    });
+}
+
+fn filled_sector(c: &mut Criterion) {
+    c.bench_function("filled_sector", |b| {
+        let object = &Sector::new(Point::new(100, 100), 100, -30.0.deg(), 150.0.deg())
+            .into_styled(PrimitiveStyle::with_fill(Gray8::new(1)));
+
+        let mut framebuffer = Framebuffer::new();
+        b.iter(|| object.draw(&mut framebuffer))
+    });
+}
+
+criterion_group!(primitives_fixed_point, arc, sector, filled_sector);
+criterion_main!(primitives_fixed_point);