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/defmt-parser/README.md

@@ -0,0 +1,42 @@
+# `defmt-parser`
+
+`defmt` ("de format", short for "deferred formatting") is a highly efficient logging framework that targets resource-constrained devices, like microcontrollers.
+
+For more details about the framework check the book at <https://defmt.ferrous-systems.com>.
+
+This library decodes `defmt` frames into Rust structures. It is mainly used by
+[`defmt-decoder`](https://crates.io/crates/defmt-decoder), and you should prefer
+using that crate to this one.
+
+## MSRV
+
+The minimum supported Rust version is 1.76 (or Ferrocene 24.05). `defmt` is tested against the latest stable Rust version and the MSRV.
+
+## Support
+
+`defmt` is part of the [Knurling] project, [Ferrous Systems]' effort at
+improving tooling used to develop for embedded systems.
+
+If you think that our work is useful, consider sponsoring it via [GitHub
+Sponsors].
+
+## 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
+licensed as above, without any additional terms or conditions.
+
+[Knurling]: https://knurling.ferrous-systems.com/
+[Ferrous Systems]: https://ferrous-systems.com/
+[GitHub Sponsors]: https://github.com/sponsors/knurling-rs