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/scopeguard/examples/readme.rs

@@ -0,0 +1,29 @@
+#[macro_use(defer)]
+extern crate scopeguard;
+
+use scopeguard::guard;
+
+fn f() {
+    defer! {
+        println!("Called at return or panic");
+    }
+    panic!();
+}
+
+use std::fs::File;
+use std::io::Write;
+
+fn g() {
+    let f = File::create("newfile.txt").unwrap();
+    let mut file = guard(f, |f| {
+        // write file at return or panic
+        let _ = f.sync_all();
+    });
+    // access the file through the scope guard itself
+    file.write_all(b"test me\n").unwrap();
+}
+
+fn main() {
+    f();
+    g();
+}