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/src/imp_cs.rs

@@ -0,0 +1,78 @@
+use core::panic::{RefUnwindSafe, UnwindSafe};
+
+use critical_section::{CriticalSection, Mutex};
+use portable_atomic::{AtomicBool, Ordering};
+
+use crate::unsync;
+
+pub(crate) struct OnceCell<T> {
+    initialized: AtomicBool,
+    // Use `unsync::OnceCell` internally since `Mutex` does not provide
+    // interior mutability and to be able to re-use `get_or_try_init`.
+    value: Mutex<unsync::OnceCell<T>>,
+}
+
+// Why do we need `T: Send`?
+// Thread A creates a `OnceCell` and shares it with
+// scoped thread B, which fills the cell, which is
+// then destroyed by A. That is, destructor observes
+// a sent value.
+unsafe impl<T: Sync + Send> Sync for OnceCell<T> {}
+unsafe impl<T: Send> Send for OnceCell<T> {}
+
+impl<T: RefUnwindSafe + UnwindSafe> RefUnwindSafe for OnceCell<T> {}
+impl<T: UnwindSafe> UnwindSafe for OnceCell<T> {}
+
+impl<T> OnceCell<T> {
+    pub(crate) const fn new() -> OnceCell<T> {
+        OnceCell { initialized: AtomicBool::new(false), value: Mutex::new(unsync::OnceCell::new()) }
+    }
+
+    pub(crate) const fn with_value(value: T) -> OnceCell<T> {
+        OnceCell {
+            initialized: AtomicBool::new(true),
+            value: Mutex::new(unsync::OnceCell::with_value(value)),
+        }
+    }
+
+    #[inline]
+    pub(crate) fn is_initialized(&self) -> bool {
+        self.initialized.load(Ordering::Acquire)
+    }
+
+    #[cold]
+    pub(crate) fn initialize<F, E>(&self, f: F) -> Result<(), E>
+    where
+        F: FnOnce() -> Result<T, E>,
+    {
+        critical_section::with(|cs| {
+            let cell = self.value.borrow(cs);
+            cell.get_or_try_init(f).map(|_| {
+                self.initialized.store(true, Ordering::Release);
+            })
+        })
+    }
+
+    /// Get the reference to the underlying value, without checking if the cell
+    /// is initialized.
+    ///
+    /// # Safety
+    ///
+    /// Caller must ensure that the cell is in initialized state, and that
+    /// the contents are acquired by (synchronized to) this thread.
+    pub(crate) unsafe fn get_unchecked(&self) -> &T {
+        debug_assert!(self.is_initialized());
+        // SAFETY: The caller ensures that the value is initialized and access synchronized.
+        self.value.borrow(CriticalSection::new()).get().unwrap_unchecked()
+    }
+
+    #[inline]
+    pub(crate) fn get_mut(&mut self) -> Option<&mut T> {
+        self.value.get_mut().get_mut()
+    }
+
+    #[inline]
+    pub(crate) fn into_inner(self) -> Option<T> {
+        self.value.into_inner().into_inner()
+    }
+}