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/esp-hal/README.md

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+# esp-hal
+
+[![Crates.io](https://img.shields.io/crates/v/esp-hal?labelColor=1C2C2E&color=C96329&logo=Rust&style=flat-square)](https://crates.io/crates/esp-hal)
+[![docs.rs](https://img.shields.io/docsrs/esp-hal?labelColor=1C2C2E&color=C96329&logo=rust&style=flat-square)](https://docs.esp-rs.org/esp-hal)
+![MSRV](https://img.shields.io/badge/MSRV-1.76-blue?labelColor=1C2C2E&style=flat-square)
+![Crates.io](https://img.shields.io/crates/l/esp-hal?labelColor=1C2C2E&style=flat-square)
+[![Matrix](https://img.shields.io/matrix/esp-rs:matrix.org?label=join%20matrix&labelColor=1C2C2E&color=BEC5C9&logo=matrix&style=flat-square)](https://matrix.to/#/#esp-rs:matrix.org)
+
+Bare-metal (`no_std`) hardware abstraction layer for Espressif devices.
+
+Implements a number of blocking and, where applicable, async traits from the various packages in the [embedded-hal] repository.
+
+For help getting started with this HAL, please refer to [The Rust on ESP Book] and the [documentation].
+
+[embedded-hal]: https://github.com/rust-embedded/embedded-hal
+[the rust on esp book]: https://docs.esp-rs.org/book/
+
+## [Documentation]
+
+[documentation]: https://docs.esp-rs.org/esp-hal/
+
+## Supported Devices
+
+|   Chip   |        Datasheet         | Technical Reference Manual |             Target             |
+| :------: | :----------------------: | :------------------------: | :----------------------------: |
+|  ESP32   |  [ESP32][32-datasheet]   |      [ESP32][32-trm]       |    `xtensa-esp32-none-elf`     |
+| ESP32-C2 | [ESP32-C2][c2-datasheet] |     [ESP32-C2][c2-trm]     | `riscv32imc-unknown-none-elf`  |
+| ESP32-C3 | [ESP32-C3][c3-datasheet] |     [ESP32-C3][c3-trm]     | `riscv32imc-unknown-none-elf`  |
+| ESP32-C6 | [ESP32-C6][c6-datasheet] |     [ESP32-C6][c6-trm]     | `riscv32imac-unknown-none-elf` |
+| ESP32-H2 | [ESP32-H2][h2-datasheet] |     [ESP32-H2][h2-trm]     | `riscv32imac-unknown-none-elf` |
+| ESP32-S2 | [ESP32-S2][s2-datasheet] |     [ESP32-S2][s2-trm]     |   `xtensa-esp32s2-none-elf`    |
+| ESP32-S3 | [ESP32-S3][s3-datasheet] |     [ESP32-S3][s3-trm]     |   `xtensa-esp32s3-none-elf`    |
+
+[32-datasheet]: https://www.espressif.com/sites/default/files/documentation/esp32_datasheet_en.pdf
+[c2-datasheet]: https://www.espressif.com/sites/default/files/documentation/esp8684_datasheet_en.pdf
+[c3-datasheet]: https://www.espressif.com/sites/default/files/documentation/esp32-c3_datasheet_en.pdf
+[c6-datasheet]: https://www.espressif.com/sites/default/files/documentation/esp32-c6_datasheet_en.pdf
+[h2-datasheet]: https://www.espressif.com/sites/default/files/documentation/esp32-h2_datasheet_en.pdf
+[s2-datasheet]: https://www.espressif.com/sites/default/files/documentation/esp32-s2_datasheet_en.pdf
+[s3-datasheet]: https://www.espressif.com/sites/default/files/documentation/esp32-s3_datasheet_en.pdf
+[32-trm]: https://www.espressif.com/sites/default/files/documentation/esp32_technical_reference_manual_en.pdf
+[c2-trm]: https://www.espressif.com/sites/default/files/documentation/esp8684_technical_reference_manual_en.pdf
+[c3-trm]: https://www.espressif.com/sites/default/files/documentation/esp32-c3_technical_reference_manual_en.pdf
+[c6-trm]: https://www.espressif.com/sites/default/files/documentation/esp32-c6_technical_reference_manual_en.pdf
+[h2-trm]: https://www.espressif.com/sites/default/files/documentation/esp32-h2_technical_reference_manual_en.pdf
+[s2-trm]: https://www.espressif.com/sites/default/files/documentation/esp32-s2_technical_reference_manual_en.pdf
+[s3-trm]: https://www.espressif.com/sites/default/files/documentation/esp32-s3_technical_reference_manual_en.pdf
+
+## Minimum Supported Rust Version (MSRV)
+
+This crate is guaranteed to compile on stable Rust 1.79 and up. It _might_
+compile with older versions but that may change in any new patch release.
+
+## 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.