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/spin/src/lazy.rs

@@ -0,0 +1,118 @@
+//! Synchronization primitives for lazy evaluation.
+//!
+//! Implementation adapted from the `SyncLazy` type of the standard library. See:
+//! <https://doc.rust-lang.org/std/lazy/struct.SyncLazy.html>
+
+use crate::{once::Once, RelaxStrategy, Spin};
+use core::{cell::Cell, fmt, ops::Deref};
+
+/// A value which is initialized on the first access.
+///
+/// This type is a thread-safe `Lazy`, and can be used in statics.
+///
+/// # Examples
+///
+/// ```
+/// use std::collections::HashMap;
+/// use spin::Lazy;
+///
+/// static HASHMAP: Lazy<HashMap<i32, String>> = Lazy::new(|| {
+///     println!("initializing");
+///     let mut m = HashMap::new();
+///     m.insert(13, "Spica".to_string());
+///     m.insert(74, "Hoyten".to_string());
+///     m
+/// });
+///
+/// fn main() {
+///     println!("ready");
+///     std::thread::spawn(|| {
+///         println!("{:?}", HASHMAP.get(&13));
+///     }).join().unwrap();
+///     println!("{:?}", HASHMAP.get(&74));
+///
+///     // Prints:
+///     //   ready
+///     //   initializing
+///     //   Some("Spica")
+///     //   Some("Hoyten")
+/// }
+/// ```
+pub struct Lazy<T, F = fn() -> T, R = Spin> {
+    cell: Once<T, R>,
+    init: Cell<Option<F>>,
+}
+
+impl<T: fmt::Debug, F, R> fmt::Debug for Lazy<T, F, R> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("Lazy")
+            .field("cell", &self.cell)
+            .field("init", &"..")
+            .finish()
+    }
+}
+
+// We never create a `&F` from a `&Lazy<T, F>` so it is fine
+// to not impl `Sync` for `F`
+// we do create a `&mut Option<F>` in `force`, but this is
+// properly synchronized, so it only happens once
+// so it also does not contribute to this impl.
+unsafe impl<T, F: Send> Sync for Lazy<T, F> where Once<T>: Sync {}
+// auto-derived `Send` impl is OK.
+
+impl<T, F, R> Lazy<T, F, R> {
+    /// Creates a new lazy value with the given initializing
+    /// function.
+    pub const fn new(f: F) -> Self {
+        Self {
+            cell: Once::new(),
+            init: Cell::new(Some(f)),
+        }
+    }
+    /// Retrieves a mutable pointer to the inner data.
+    ///
+    /// This is especially useful when interfacing with low level code or FFI where the caller
+    /// explicitly knows that it has exclusive access to the inner data. Note that reading from
+    /// this pointer is UB until initialized or directly written to.
+    pub fn as_mut_ptr(&self) -> *mut T {
+        self.cell.as_mut_ptr()
+    }
+}
+
+impl<T, F: FnOnce() -> T, R: RelaxStrategy> Lazy<T, F, R> {
+    /// Forces the evaluation of this lazy value and
+    /// returns a reference to result. This is equivalent
+    /// to the `Deref` impl, but is explicit.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// use spin::Lazy;
+    ///
+    /// let lazy = Lazy::new(|| 92);
+    ///
+    /// assert_eq!(Lazy::force(&lazy), &92);
+    /// assert_eq!(&*lazy, &92);
+    /// ```
+    pub fn force(this: &Self) -> &T {
+        this.cell.call_once(|| match this.init.take() {
+            Some(f) => f(),
+            None => panic!("Lazy instance has previously been poisoned"),
+        })
+    }
+}
+
+impl<T, F: FnOnce() -> T, R: RelaxStrategy> Deref for Lazy<T, F, R> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        Self::force(self)
+    }
+}
+
+impl<T: Default, R> Default for Lazy<T, fn() -> T, R> {
+    /// Creates a new lazy value using `Default` as the initializing function.
+    fn default() -> Self {
+        Self::new(T::default)
+    }
+}