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/serde/src/private/mod.rs

@@ -0,0 +1,18 @@
+#[cfg(not(no_serde_derive))]
+pub mod de;
+#[cfg(not(no_serde_derive))]
+pub mod ser;
+
+pub use crate::lib::clone::Clone;
+pub use crate::lib::convert::{From, Into, TryFrom};
+pub use crate::lib::default::Default;
+pub use crate::lib::fmt::{self, Formatter};
+pub use crate::lib::marker::PhantomData;
+pub use crate::lib::option::Option::{self, None, Some};
+pub use crate::lib::ptr;
+pub use crate::lib::result::Result::{self, Err, Ok};
+
+pub use crate::serde_core_private::string::from_utf8_lossy;
+
+#[cfg(any(feature = "alloc", feature = "std"))]
+pub use crate::lib::{ToString, Vec};