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/bitflags-1.3.2/tests/compile.rs

@@ -0,0 +1,63 @@
+use std::{
+    fs,
+    ffi::OsStr,
+    io,
+    path::Path,
+};
+
+use walkdir::WalkDir;
+
+#[test]
+fn fail() {
+    prepare_stderr_files("tests/compile-fail").unwrap();
+
+    let t = trybuild::TestCases::new();
+    t.compile_fail("tests/compile-fail/**/*.rs");
+}
+
+#[test]
+fn pass() {
+    let t = trybuild::TestCases::new();
+    t.pass("tests/compile-pass/**/*.rs");
+}
+
+// Compiler messages may change between versions
+// We don't want to have to track these too closely for `bitflags`, but
+// having some message to check makes sure user-facing errors are sensical.
+// 
+// The approach we use is to run the test on all compilers, but only check stderr
+// output on beta (which is the next stable release). We do this by default ignoring
+// any `.stderr` files in the `compile-fail` directory, and copying `.stderr.beta` files
+// when we happen to be running on a beta compiler.
+fn prepare_stderr_files(path: impl AsRef<Path>) -> io::Result<()> {
+    for entry in WalkDir::new(path) {
+        let entry = entry?;
+
+        if entry.path().extension().and_then(OsStr::to_str) == Some("beta") {
+            let renamed = entry.path().with_extension("");
+
+            // Unconditionally remove a corresponding `.stderr` file for a `.stderr.beta`
+            // file if it exists. On `beta` compilers, we'll recreate it. On other compilers,
+            // we don't want to end up checking it anyways.
+            if renamed.exists() {
+                fs::remove_file(&renamed)?;
+            }
+
+            rename_beta_stderr(entry.path(), renamed)?;
+        }
+    }
+
+    Ok(())
+}
+
+#[rustversion::beta]
+fn rename_beta_stderr(from: impl AsRef<Path>, to: impl AsRef<Path>) -> io::Result<()> {
+    fs::copy(from, to)?;
+
+    Ok(())
+}
+
+#[rustversion::not(beta)]
+fn rename_beta_stderr(_: impl AsRef<Path>, _: impl AsRef<Path>) -> io::Result<()> {
+    Ok(())
+}