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/once_cell/tests/it/race.rs

@@ -0,0 +1,191 @@
+#[cfg(feature = "std")]
+use std::sync::Barrier;
+use std::{
+    num::NonZeroUsize,
+    sync::atomic::{AtomicUsize, Ordering::SeqCst},
+    thread::scope,
+};
+
+use once_cell::race::{OnceBool, OnceNonZeroUsize, OnceRef};
+
+#[test]
+fn once_non_zero_usize_smoke_test() {
+    let cnt = AtomicUsize::new(0);
+    let cell = OnceNonZeroUsize::new();
+    let val = NonZeroUsize::new(92).unwrap();
+    scope(|s| {
+        s.spawn(|| {
+            assert_eq!(
+                cell.get_or_init(|| {
+                    cnt.fetch_add(1, SeqCst);
+                    val
+                }),
+                val
+            );
+            assert_eq!(cnt.load(SeqCst), 1);
+
+            assert_eq!(
+                cell.get_or_init(|| {
+                    cnt.fetch_add(1, SeqCst);
+                    val
+                }),
+                val
+            );
+            assert_eq!(cnt.load(SeqCst), 1);
+        });
+    });
+    assert_eq!(cell.get(), Some(val));
+    assert_eq!(cnt.load(SeqCst), 1);
+}
+
+#[test]
+fn once_non_zero_usize_set() {
+    let val1 = NonZeroUsize::new(92).unwrap();
+    let val2 = NonZeroUsize::new(62).unwrap();
+
+    let cell = OnceNonZeroUsize::new();
+
+    assert!(cell.set(val1).is_ok());
+    assert_eq!(cell.get(), Some(val1));
+
+    assert!(cell.set(val2).is_err());
+    assert_eq!(cell.get(), Some(val1));
+}
+
+#[cfg(feature = "std")]
+#[test]
+fn once_non_zero_usize_first_wins() {
+    let val1 = NonZeroUsize::new(92).unwrap();
+    let val2 = NonZeroUsize::new(62).unwrap();
+
+    let cell = OnceNonZeroUsize::new();
+
+    let b1 = Barrier::new(2);
+    let b2 = Barrier::new(2);
+    let b3 = Barrier::new(2);
+    scope(|s| {
+        s.spawn(|| {
+            let r1 = cell.get_or_init(|| {
+                b1.wait();
+                b2.wait();
+                val1
+            });
+            assert_eq!(r1, val1);
+            b3.wait();
+        });
+        b1.wait();
+        s.spawn(|| {
+            let r2 = cell.get_or_init(|| {
+                b2.wait();
+                b3.wait();
+                val2
+            });
+            assert_eq!(r2, val1);
+        });
+    });
+
+    assert_eq!(cell.get(), Some(val1));
+}
+
+#[test]
+fn once_bool_smoke_test() {
+    let cnt = AtomicUsize::new(0);
+    let cell = OnceBool::new();
+    scope(|s| {
+        s.spawn(|| {
+            assert_eq!(
+                cell.get_or_init(|| {
+                    cnt.fetch_add(1, SeqCst);
+                    false
+                }),
+                false
+            );
+            assert_eq!(cnt.load(SeqCst), 1);
+
+            assert_eq!(
+                cell.get_or_init(|| {
+                    cnt.fetch_add(1, SeqCst);
+                    false
+                }),
+                false
+            );
+            assert_eq!(cnt.load(SeqCst), 1);
+        });
+    });
+    assert_eq!(cell.get(), Some(false));
+    assert_eq!(cnt.load(SeqCst), 1);
+}
+
+#[test]
+fn once_bool_set() {
+    let cell = OnceBool::new();
+
+    assert!(cell.set(false).is_ok());
+    assert_eq!(cell.get(), Some(false));
+
+    assert!(cell.set(true).is_err());
+    assert_eq!(cell.get(), Some(false));
+}
+
+#[test]
+fn once_bool_get_or_try_init() {
+    let cell = OnceBool::new();
+
+    let result1: Result<bool, ()> = cell.get_or_try_init(|| Ok(true));
+    let result2: Result<bool, ()> = cell.get_or_try_init(|| Ok(false));
+    assert_eq!(result1, Ok(true));
+    assert_eq!(result2, Ok(true));
+
+    let cell = OnceBool::new();
+
+    let result3: Result<bool, ()> = cell.get_or_try_init(|| Err(()));
+    assert_eq!(result3, Err(()));
+}
+
+#[test]
+fn once_ref_smoke_test() {
+    let cnt: AtomicUsize = AtomicUsize::new(0);
+    let cell: OnceRef<'_, &str> = OnceRef::new();
+    scope(|s| {
+        s.spawn(|| {
+            assert_eq!(
+                cell.get_or_init(|| {
+                    cnt.fetch_add(1, SeqCst);
+                    &"false"
+                }),
+                &"false"
+            );
+            assert_eq!(cnt.load(SeqCst), 1);
+
+            assert_eq!(
+                cell.get_or_init(|| {
+                    cnt.fetch_add(1, SeqCst);
+                    &"false"
+                }),
+                &"false"
+            );
+            assert_eq!(cnt.load(SeqCst), 1);
+        });
+    });
+    assert_eq!(cell.get(), Some(&"false"));
+    assert_eq!(cnt.load(SeqCst), 1);
+}
+
+#[test]
+fn once_ref_set() {
+    let cell: OnceRef<'_, &str> = OnceRef::new();
+
+    assert!(cell.set(&"false").is_ok());
+    assert_eq!(cell.get(), Some(&"false"));
+
+    assert!(cell.set(&"true").is_err());
+    assert_eq!(cell.get(), Some(&"false"));
+}
+
+#[test]
+fn get_unchecked() {
+    let cell = OnceNonZeroUsize::new();
+    cell.set(NonZeroUsize::new(92).unwrap()).unwrap();
+    let value = unsafe { cell.get_unchecked() };
+    assert_eq!(value, NonZeroUsize::new(92).unwrap());
+}