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/void/Cargo.toml

@@ -0,0 +1,15 @@
+[package]
+
+name = "void"
+version = "1.0.2"
+authors = ["Jonathan Reem <jonathan.reem@gmail.com>"]
+repository = "https://github.com/reem/rust-void.git"
+description = "The uninhabited void type for use in statically impossible cases."
+readme = "README.md"
+license = "MIT"
+
+[features]
+
+default = ["std"]
+std = []
+