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/embedded-hal-async/build.rs

@@ -0,0 +1,18 @@
+use std::env;
+use std::ffi::OsString;
+use std::process::Command;
+
+fn main() {
+    println!("cargo:rerun-if-changed=build.rs");
+
+    let rustc = env::var_os("RUSTC").unwrap_or_else(|| OsString::from("rustc"));
+
+    let output = Command::new(rustc)
+        .arg("--version")
+        .output()
+        .expect("failed to run `rustc --version`");
+
+    if String::from_utf8_lossy(&output.stdout).contains("nightly") {
+        println!("cargo:rustc-cfg=nightly");
+    }
+}