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/r0/README.md

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+# r0
+
+[![Build status](https://api.travis-ci.org/rust-embedded/r0.svg?branch=master)](https://travis-ci.org/rust-embedded/r0)
+[![crates.io](https://img.shields.io/crates/d/r0.svg)](https://crates.io/crates/r0)
+[![crates.io](https://img.shields.io/crates/v/r0.svg)](https://crates.io/crates/r0)
+
+Memory initialization code written in Rust.
+
+This crate is for bare metal systems where there is no ELF loader or OS to take care of
+initializing RAM for the program.
+
+r0 is not meant to be used by user applications directly. Instead, it is most often used by
+embedded runtime crates, like:
+
+* [cortex-m-rt](https://github.com/rust-embedded/cortex-m-rt)
+* [riscv-rt](https://github.com/rust-embedded/riscv-rt)
+* [msp430-rt](https://github.com/rust-embedded/msp430-rt)
+
+The r0 crate provides similar functionality to [crt0](https://en.wikipedia.org/wiki/Crt0) in the C
+runtime.
+
+This project is developed and maintained by the [Cortex-A, Cortex-M, Cortex-R, MSP430, and RISCV
+teams][teams].
+
+## [Documentation](https://docs.rs/r0)
+
+## License
+
+Licensed under either of
+
+* Apache License, Version 2.0 ([LICENSE-APACHE](LICENSE-APACHE) or
+  <https://www.apache.org/licenses/LICENSE-2.0>)
+* MIT license ([LICENSE-MIT](LICENSE-MIT) or <https://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.
+
+## Code of Conduct
+
+Contribution to this crate is organized under the terms of the [Rust Code of
+Conduct][CoC], the maintainers of this crate, the [Cortex-A, Cortex-M,
+Cortex-R, MSP430, and RISCV teams][teams], promise to intervene to uphold that
+code of conduct.
+
+[CoC]: CODE_OF_CONDUCT.md
+[teams]: https://github.com/rust-embedded/wg#organization