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-hal-0.2.7/README.md

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+# `embedded-hal`
+
+>  A Hardware Abstraction Layer (HAL) for embedded systems
+
+This project is developed and maintained by the [HAL team][team].
+
+## [API reference]
+
+[API reference]: https://docs.rs/embedded-hal
+
+## How-to: add a new trait
+
+This is the suggested approach to adding a new trait to `embedded-hal`
+
+### Discussion
+
+Ideally, before proposing a new trait, or set of traits, you should create an issue where the use
+cases and requirements of the trait(s) are discussed.
+
+These issues will be labeled as `discussion`s in the issue tracker.
+
+### Proposing a trait
+
+Once there's consensus on the requirements of the trait(s) a new issue, or a PR, with a proposal
+should be opened. The proposal should include the actual trait definitions as well as a link to the
+issue with previous discussion, if there was one.
+
+If the proposal includes more than one alternative then there should be further discussion to try to
+single out the best alternative.
+
+These issues / PRs will be labeled as `proposal`s in the issue tracker.
+
+### Testing period
+
+If there are no objections to the proposal the new trait(s) will land behind the "unproven" Cargo
+feature and an issue about the new trait(s) will be created. If the proposal includes several
+alternatives and a single one couldn't be chosen as the best then each alternative will land behind
+a different Cargo feature, e.g. "alt1" or "alt2".
+
+The traits will undergo a testing period before they move into the set of proven traits. During
+this period users are encouraged to try to implement the unproven traits for their platforms and to
+build drivers on top of them. Problems implementing the trait(s) as well as successful
+implementations should be reported on the corresponding issue.
+
+To leave the unproven state at least *two* implementations of the trait(s) for different platforms
+(ideally, the two platforms should be from different vendors) and *one* generic driver built on top
+of the trait(s), or alternatively one demo program that exercises the trait (via generic function /
+trait object), *should* be demonstrated. If, instead, reports indicate that the proposed trait(s)
+can't be implemented for a certain platform then the trait(s) will be removed and we'll go back to
+the drawing board.
+
+Issues used to track unproven APIs will be labeled as `unproven-api`s in the issue tracker and they
+may also include the labels `needs-impl` and `needs-driver` to signal what's required for them to
+move to the set of proven traits.
+
+## Implementations and drivers
+
+For a list of `embedded-hal` implementations and driver crates check the [awesome-embedded-rust]
+list.
+
+[awesome-embedded-rust]: https://github.com/rust-embedded/awesome-embedded-rust#driver-crates
+
+## 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.
+
+## Code of Conduct
+
+Contribution to this crate is organized under the terms of the [Rust Code of
+Conduct][CoC], the maintainer of this crate, the [HAL team][team], promises
+to intervene to uphold that code of conduct.
+
+[CoC]: CODE_OF_CONDUCT.md
+[team]: https://github.com/rust-embedded/wg#the-hal-team