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/anyhow/tests/drop/mod.rs

@@ -0,0 +1,53 @@
+#![allow(clippy::module_name_repetitions)]
+
+use std::error::Error as StdError;
+use std::fmt::{self, Display};
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::Arc;
+
+#[derive(Debug)]
+pub struct Flag {
+    atomic: Arc<AtomicBool>,
+}
+
+impl Flag {
+    pub fn new() -> Self {
+        Flag {
+            atomic: Arc::new(AtomicBool::new(false)),
+        }
+    }
+
+    pub fn get(&self) -> bool {
+        self.atomic.load(Ordering::Relaxed)
+    }
+}
+
+#[derive(Debug)]
+pub struct DetectDrop {
+    has_dropped: Flag,
+}
+
+impl DetectDrop {
+    pub fn new(has_dropped: &Flag) -> Self {
+        DetectDrop {
+            has_dropped: Flag {
+                atomic: Arc::clone(&has_dropped.atomic),
+            },
+        }
+    }
+}
+
+impl StdError for DetectDrop {}
+
+impl Display for DetectDrop {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        write!(f, "oh no!")
+    }
+}
+
+impl Drop for DetectDrop {
+    fn drop(&mut self) {
+        let already_dropped = self.has_dropped.atomic.swap(true, Ordering::Relaxed);
+        assert!(!already_dropped);
+    }
+}