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/object/build.rs

@@ -0,0 +1,27 @@
+use std::process::Command;
+use std::str;
+
+fn main() {
+    // Temporary check to see if the rustc version >= 1.81 in which case the
+    // `Error` trait is always available. This is temporary because in the
+    // future the MSRV of this crate will be beyond 1.81 in which case this
+    // build script can be deleted.
+    let minor = rustc_minor_version().unwrap_or(0);
+    if minor >= 81 {
+        println!("cargo:rustc-cfg=core_error");
+    }
+    if minor >= 80 {
+        println!("cargo:rustc-check-cfg=cfg(core_error)");
+    }
+}
+
+fn rustc_minor_version() -> Option<u32> {
+    let rustc = std::env::var("RUSTC").unwrap();
+    let output = Command::new(rustc).arg("--version").output().ok()?;
+    let version = str::from_utf8(&output.stdout).ok()?;
+    let mut pieces = version.split('.');
+    if pieces.next() != Some("rustc 1") {
+        return None;
+    }
+    pieces.next()?.parse().ok()
+}