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/cfg-if/CHANGELOG.md

@@ -0,0 +1,29 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+## [1.0.4](https://github.com/rust-lang/cfg-if/compare/v1.0.3...v1.0.4) - 2025-10-15
+
+- Support `cfg(true)` and `cfg(false)` ([#99](https://github.com/rust-lang/cfg-if/pull/99))
+- Set and test a MSRV of 1.32
+- Have a single top-level rule
+
+## [1.0.3](https://github.com/rust-lang/cfg-if/compare/v1.0.2...v1.0.3) - 2025-08-19
+
+- Revert "Remove `@__identity` rule."
+
+## [1.0.2](https://github.com/rust-lang/cfg-if/compare/v1.0.1...v1.0.2) - 2025-08-19
+
+- Remove `@__identity` rule.
+
+## [1.0.1](https://github.com/rust-lang/cfg-if/compare/v1.0.0...v1.0.1) - 2025-06-09
+
+- Remove `compiler-builtins` from `rustc-dep-of-std` dependencies
+- Remove redundant configuration from Cargo.toml
+- More readable formatting and identifier names. ([#39](https://github.com/rust-lang/cfg-if/pull/39))
+- Add expanded example to readme ([#38](https://github.com/rust-lang/cfg-if/pull/38))