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/log/tests/integration.rs

@@ -0,0 +1,101 @@
+#![allow(dead_code, unused_imports)]
+
+use log::{debug, error, info, trace, warn, Level, LevelFilter, Log, Metadata, Record};
+use std::sync::{Arc, Mutex};
+
+struct State {
+    last_log_level: Mutex<Option<Level>>,
+    last_log_location: Mutex<Option<u32>>,
+}
+
+struct Logger(Arc<State>);
+
+impl Log for Logger {
+    fn enabled(&self, _: &Metadata) -> bool {
+        true
+    }
+
+    fn log(&self, record: &Record) {
+        *self.0.last_log_level.lock().unwrap() = Some(record.level());
+        *self.0.last_log_location.lock().unwrap() = record.line();
+    }
+    fn flush(&self) {}
+}
+
+#[test]
+fn test_integration() {
+    // These tests don't really make sense when static
+    // max level filtering is applied
+    #[cfg(not(any(
+        feature = "max_level_off",
+        feature = "max_level_error",
+        feature = "max_level_warn",
+        feature = "max_level_info",
+        feature = "max_level_debug",
+        feature = "max_level_trace",
+        feature = "release_max_level_off",
+        feature = "release_max_level_error",
+        feature = "release_max_level_warn",
+        feature = "release_max_level_info",
+        feature = "release_max_level_debug",
+        feature = "release_max_level_trace",
+    )))]
+    {
+        let me = Arc::new(State {
+            last_log_level: Mutex::new(None),
+            last_log_location: Mutex::new(None),
+        });
+        let a = me.clone();
+        let logger = Logger(me);
+
+        test_filter(&logger, &a, LevelFilter::Off);
+        test_filter(&logger, &a, LevelFilter::Error);
+        test_filter(&logger, &a, LevelFilter::Warn);
+        test_filter(&logger, &a, LevelFilter::Info);
+        test_filter(&logger, &a, LevelFilter::Debug);
+        test_filter(&logger, &a, LevelFilter::Trace);
+
+        test_line_numbers(&logger, &a);
+    }
+}
+
+fn test_filter(logger: &dyn Log, a: &State, filter: LevelFilter) {
+    // tests to ensure logs with a level beneath 'max_level' are filtered out
+    log::set_max_level(filter);
+    error!(logger: logger, "");
+    last(a, t(Level::Error, filter));
+    warn!(logger: logger, "");
+    last(a, t(Level::Warn, filter));
+    info!(logger: logger, "");
+    last(a, t(Level::Info, filter));
+    debug!(logger: logger, "");
+    last(a, t(Level::Debug, filter));
+    trace!(logger: logger, "");
+    last(a, t(Level::Trace, filter));
+
+    fn t(lvl: Level, filter: LevelFilter) -> Option<Level> {
+        if lvl <= filter {
+            Some(lvl)
+        } else {
+            None
+        }
+    }
+    fn last(state: &State, expected: Option<Level>) {
+        let lvl = state.last_log_level.lock().unwrap().take();
+        assert_eq!(lvl, expected);
+    }
+}
+
+fn test_line_numbers(logger: &dyn Log, state: &State) {
+    log::set_max_level(LevelFilter::Trace);
+
+    info!(logger: logger, ""); // ensure check_line function follows log macro
+    check_log_location(state);
+
+    #[track_caller]
+    fn check_log_location(state: &State) {
+        let location = std::panic::Location::caller().line(); // get function calling location
+        let line_number = state.last_log_location.lock().unwrap().take().unwrap(); // get location of most recent log
+        assert_eq!(line_number, location - 1);
+    }
+}