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/rustversion/src/version.rs

@@ -0,0 +1,18 @@
+#![allow(dead_code)]
+
+use crate::date::Date;
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub struct Version {
+    pub minor: u16,
+    pub patch: u16,
+    pub channel: Channel,
+}
+
+#[derive(Copy, Clone, Debug, PartialEq)]
+pub enum Channel {
+    Stable,
+    Beta,
+    Nightly(Date),
+    Dev,
+}