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/heapless/README.md

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+[![crates.io](https://img.shields.io/crates/v/heapless.svg)](https://crates.io/crates/heapless)
+[![crates.io](https://img.shields.io/crates/d/heapless.svg)](https://crates.io/crates/heapless)
+
+# `heapless`
+
+> `static` friendly data structures that don't require dynamic memory allocation
+
+This project is developed and maintained by the [libs team].
+
+## [Documentation](https://docs.rs/heapless/latest/heapless)
+
+## [Change log](CHANGELOG.md)
+
+## Tests
+
+``` console
+$ # run all
+$ cargo test --features serde
+
+$ # run only for example histbuf tests
+$ cargo test histbuf --features serde
+```
+
+## 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.
+
+[libs team]: https://github.com/rust-embedded/wg#the-libs-team