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/bytemuck/tests/offset_of_tests.rs

@@ -0,0 +1,60 @@
+#![allow(clippy::disallowed_names)]
+use bytemuck::{offset_of, Zeroable};
+
+#[test]
+fn test_offset_of_vertex() {
+  #[repr(C)]
+  struct Vertex {
+    pos: [f32; 2],
+    uv: [u16; 2],
+    color: [u8; 4],
+  }
+  unsafe impl Zeroable for Vertex {}
+
+  let pos = offset_of!(Zeroable::zeroed(), Vertex, pos);
+  let uv = offset_of!(Zeroable::zeroed(), Vertex, uv);
+  let color = offset_of!(Zeroable::zeroed(), Vertex, color);
+
+  assert_eq!(pos, 0);
+  assert_eq!(uv, 8);
+  assert_eq!(color, 12);
+}
+
+#[test]
+fn test_offset_of_foo() {
+  #[derive(Default)]
+  struct Foo {
+    a: u8,
+    b: &'static str,
+    c: i32,
+  }
+
+  let a_offset = offset_of!(Default::default(), Foo, a);
+  let b_offset = offset_of!(Default::default(), Foo, b);
+  let c_offset = offset_of!(Default::default(), Foo, c);
+
+  assert_ne!(a_offset, b_offset);
+  assert_ne!(b_offset, c_offset);
+  // We can't check against hardcoded values for a repr(Rust) type,
+  // but prove to ourself this way.
+
+  let foo = Foo::default();
+  // Note: offsets are in bytes.
+  let as_bytes = &foo as *const _ as *const u8;
+
+  // we're using wrapping_offset here because it's not worth
+  // the unsafe block, but it would be valid to use `add` instead,
+  // as it cannot overflow.
+  assert_eq!(
+    &foo.a as *const _ as usize,
+    as_bytes.wrapping_add(a_offset) as usize
+  );
+  assert_eq!(
+    &foo.b as *const _ as usize,
+    as_bytes.wrapping_add(b_offset) as usize
+  );
+  assert_eq!(
+    &foo.c as *const _ as usize,
+    as_bytes.wrapping_add(c_offset) as usize
+  );
+}