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/vcell/CHANGELOG.md

@@ -0,0 +1,34 @@
+# Change Log
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](http://keepachangelog.com/)
+and this project adheres to [Semantic Versioning](http://semver.org/).
+
+## [Unreleased]
+
+## [v0.1.3] - 2021-01-01
+
+- Added `#[repr(transparent)]` to follow the `core::cell::Cell`
+
+## [v0.1.2] - 2019-08-29
+
+### Added
+
+- Added back the `const-fn` feature, but unused for backward compatibility
+
+## [v0.1.1] - 2019-08-29
+
+### Added
+
+- Added `.as_ptr()`
+- `new()` now const by default
+
+## [v0.1.0] - 2017-03-08
+
+- Initial release
+
+[Unreleased]: https://github.com/japaric/vcell/compare/v0.1.3...HEAD
+[v0.1.3]: https://github.com/japaric/vcell/compare/v0.1.2...v0.1.3
+[v0.1.2]: https://github.com/japaric/vcell/compare/v0.1.1...v0.1.2
+[v0.1.1]: https://github.com/japaric/vcell/compare/v0.1.0...v0.1.1