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/void/README.md

@@ -0,0 +1,39 @@
+# Void
+
+> The uninhabited void type for use in statically impossible cases.
+
+## [Documentation](https://crates.fyi/crates/void/1.0.1)
+
+The uninhabited type, `enum Void { }` is useful in dealing with cases you
+know to be impossible. For instance, if you are implementing a trait which
+allows for error checking, but your case always succeeds, you can mark the
+error case or type as `Void`, signaling to the compiler it can never happen.
+
+This crate also comes packed with a few traits offering extension methods to
+`Result<T, Void>` and `Result<Void, T>`.
+
+## Usage
+
+Use the crates.io repository; add this to your `Cargo.toml` along
+with the rest of your dependencies:
+
+```toml
+[dependencies]
+void = "1"
+```
+
+Then, use `Void` in your crate:
+
+```rust
+extern crate void;
+use void::Void;
+```
+
+## Author
+
+[Jonathan Reem](https://medium.com/@jreem) is the primary author and maintainer of void.
+
+## License
+
+MIT
+