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/document-features/README.md

@@ -0,0 +1,43 @@
+# Document your crate's feature flags
+
+[![Crates.io](https://img.shields.io/crates/v/document-features)](https://crates.io/crates/document-features)
+[![Documentation](https://docs.rs/document-features/badge.svg)](https://docs.rs/document-features/)
+
+This crate provides a macro that extracts documentation comments from Cargo.toml
+
+To use this crate, add `#![doc = document_features::document_features!()]` in your crate documentation.
+The `document_features!()` macro reads your `Cargo.toml` file, extracts feature comments and generates
+a markdown string for your documentation.
+
+Use `## ` and `#! ` comments in your Cargo.toml to document features, for example:
+
+```toml
+[dependencies]
+document-features = "0.2"
+## ...
+
+[features]
+## The foo feature enables the `foo` functions
+foo = []
+## The bar feature enables the [`bar`] module
+bar = []
+
+#! ### Experimental features
+#! The following features are experimental
+
+## Activate the fusion reactor
+fusion = []
+```
+
+These comments keep the feature definition and documentation next to each other, and they are then
+rendered into your crate documentation.
+
+Check out the [documentation](https://docs.rs/document-features/) for more details.
+
+## Contributions
+
+Contributions are welcome. We accept pull requests and bug reports.
+
+## License
+
+MIT OR Apache-2.0