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/az/build.rs

@@ -0,0 +1,89 @@
+// Copyright © 2019–2021 Trevor Spiteri
+
+// Copying and distribution of this file, with or without
+// modification, are permitted in any medium without royalty provided
+// the copyright notice and this notice are preserved. This file is
+// offered as-is, without any warranty.
+
+use std::{
+    env,
+    ffi::OsString,
+    fs::{self, File},
+    io::{Result as IoResult, Write},
+    path::{Path, PathBuf},
+    process::{Command, Stdio},
+};
+
+fn main() {
+    check_feature("track_caller", TRY_TRACK_CALLER);
+}
+
+fn check_feature(name: &str, contents: &str) {
+    let rustc = cargo_env("RUSTC");
+    let out_dir = PathBuf::from(cargo_env("OUT_DIR"));
+
+    let try_dir = out_dir.join(format!("try_{}", name));
+    let filename = format!("try_{}.rs", name);
+    create_dir_or_panic(&try_dir);
+    println!("$ cd {:?}", try_dir);
+    create_file_or_panic(&try_dir.join(&filename), contents);
+    let mut cmd = Command::new(&rustc);
+    cmd.current_dir(&try_dir)
+        .stdout(Stdio::null())
+        .stderr(Stdio::null())
+        .args(&[&*filename, "--emit=dep-info,metadata"]);
+    println!("$ {:?} >& /dev/null", cmd);
+    let status = cmd
+        .status()
+        .unwrap_or_else(|_| panic!("Unable to execute: {:?}", cmd));
+    if status.success() {
+        println!("cargo:rustc-cfg={}", name);
+    }
+    if cfg!(target_os = "windows") {
+        // remove_dir_all started to fail on rustc 1.64.0-nightly
+        let _ = remove_dir(&try_dir);
+    } else {
+        remove_dir_or_panic(&try_dir);
+    }
+}
+
+fn cargo_env(name: &str) -> OsString {
+    env::var_os(name)
+        .unwrap_or_else(|| panic!("environment variable not found: {}, please use cargo", name))
+}
+
+fn remove_dir(dir: &Path) -> IoResult<()> {
+    if !dir.exists() {
+        return Ok(());
+    }
+    assert!(dir.is_dir(), "Not a directory: {:?}", dir);
+    println!("$ rm -r {:?}", dir);
+    fs::remove_dir_all(dir)
+}
+
+fn remove_dir_or_panic(dir: &Path) {
+    remove_dir(dir).unwrap_or_else(|_| panic!("Unable to remove directory: {:?}", dir));
+}
+
+fn create_dir(dir: &Path) -> IoResult<()> {
+    println!("$ mkdir -p {:?}", dir);
+    fs::create_dir_all(dir)
+}
+
+fn create_dir_or_panic(dir: &Path) {
+    create_dir(dir).unwrap_or_else(|_| panic!("Unable to create directory: {:?}", dir));
+}
+
+fn create_file_or_panic(filename: &Path, contents: &str) {
+    println!("$ printf '%s' {:?}... > {:?}", &contents[0..10], filename);
+    let mut file =
+        File::create(filename).unwrap_or_else(|_| panic!("Unable to create file: {:?}", filename));
+    file.write_all(contents.as_bytes())
+        .unwrap_or_else(|_| panic!("Unable to write to file: {:?}", filename));
+}
+
+const TRY_TRACK_CALLER: &str = r#"// try_track_caller.rs
+#[track_caller]
+fn _tracked() {}
+fn main() {}
+"#;