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/autocfg/tests/support/mod.rs

@@ -0,0 +1,21 @@
+use std::borrow::Cow;
+use std::env;
+use std::path::{Path, PathBuf};
+
+/// The directory containing this test binary.
+pub fn exe_dir() -> PathBuf {
+    let exe = env::current_exe().unwrap();
+    exe.parent().unwrap().to_path_buf()
+}
+
+/// The directory to use for test probes.
+pub fn out_dir() -> Cow<'static, Path> {
+    if let Some(tmpdir) = option_env!("CARGO_TARGET_TMPDIR") {
+        Cow::Borrowed(tmpdir.as_ref())
+    } else if let Some(tmpdir) = env::var_os("TESTS_TARGET_DIR") {
+        Cow::Owned(tmpdir.into())
+    } else {
+        // Use the same path as this test binary.
+        Cow::Owned(exe_dir())
+    }
+}