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/enumset/README.md

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+# enumset
+
+[![Build Status](https://github.com/Lymia/enumset/actions/workflows/test.yml/badge.svg)](https://github.com/Lymia/enumset/actions/workflows/test.yml)
+[![Latest Version](https://img.shields.io/crates/v/enumset.svg)](https://crates.io/crates/enumset)
+![Requires rustc 1.61+](https://img.shields.io/badge/rustc-1.61+-red.svg)
+[![Rust Documentation](https://img.shields.io/badge/api-rustdoc-blue.svg)](https://docs.rs/enumset)
+
+A library for defining enums that can be used in compact bit sets. It supports
+`serde` and `#[no_std]` environments, and has basic support for using EnumSets
+in constants.
+
+See [the documentation](https://docs.rs/enumset) for more information.
+
+# License
+
+This project is 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 enumset by you, as defined in the Apache-2.0 license, shall be
+dual licensed as above, without any additional terms or conditions.