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/bare-metal-0.2.5/build.rs

@@ -0,0 +1,9 @@
+extern crate rustc_version;
+
+fn main() {
+    let vers = rustc_version::version().unwrap();
+
+    if vers.major == 1 && vers.minor < 31 {
+        println!("cargo:rustc-cfg=unstable_const_fn")
+    }
+}