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-io-async/README.md

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+[![crates.io](https://img.shields.io/crates/d/embedded-io-async.svg)](https://crates.io/crates/embedded-io-async)
+[![crates.io](https://img.shields.io/crates/v/embedded-io-async.svg)](https://crates.io/crates/embedded-io-async)
+[![Documentation](https://docs.rs/embedded-io-async/badge.svg)](https://docs.rs/embedded-io-async)
+
+# `embedded-io-async`
+
+Async IO traits for embedded systems.
+
+This crate contains asynchronous versions of the [`embedded-io`](https://crates.io/crates/embedded-io) traits and shares its scope and design goals.
+
+This project is developed and maintained by the [HAL team](https://github.com/rust-embedded/wg#the-hal-team).
+
+## Optional Cargo features
+
+- **`std`**: Adds `From` impls to convert to/from `std::io` structs, adds `std::error::Error` impls.
+- **`alloc`**: Adds blanket impls for `Box`, adds `Write` impl to `Vec`.
+- **`defmt-03`**: Derive `defmt::Format` from `defmt` 0.3 for enums and structs.
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate requires Rust nightly newer than `nightly-2022-11-22`, due to requiring support for
+`async fn` in traits (AFIT), which is not stable yet. 
+
+Keep in mind Rust nightlies can make backwards-incompatible changes to unstable features
+at any time.
+
+## License
+
+Licensed under either of
+
+- Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  <http://www.apache.org/licenses/LICENSE-2.0>)
+- MIT license ([LICENSE-MIT](LICENSE-MIT) or <http://opensource.org/licenses/MIT>)
+
+at your option.
+
+### Contribution
+
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.