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/memchr/src/ext.rs

@@ -0,0 +1,54 @@
+/// A trait for adding some helper routines to pointers.
+pub(crate) trait Pointer {
+    /// Returns the distance, in units of `T`, between `self` and `origin`.
+    ///
+    /// # Safety
+    ///
+    /// Same as `ptr::offset_from` in addition to `self >= origin`.
+    unsafe fn distance(self, origin: Self) -> usize;
+
+    /// Casts this pointer to `usize`.
+    ///
+    /// Callers should not convert the `usize` back to a pointer if at all
+    /// possible. (And if you believe it's necessary, open an issue to discuss
+    /// why. Otherwise, it has the potential to violate pointer provenance.)
+    /// The purpose of this function is just to be able to do arithmetic, i.e.,
+    /// computing offsets or alignments.
+    fn as_usize(self) -> usize;
+}
+
+impl<T> Pointer for *const T {
+    unsafe fn distance(self, origin: *const T) -> usize {
+        // TODO: Replace with `ptr::sub_ptr` once stabilized.
+        usize::try_from(self.offset_from(origin)).unwrap_unchecked()
+    }
+
+    fn as_usize(self) -> usize {
+        self as usize
+    }
+}
+
+impl<T> Pointer for *mut T {
+    unsafe fn distance(self, origin: *mut T) -> usize {
+        (self as *const T).distance(origin as *const T)
+    }
+
+    fn as_usize(self) -> usize {
+        (self as *const T).as_usize()
+    }
+}
+
+/// A trait for adding some helper routines to raw bytes.
+#[cfg(test)]
+pub(crate) trait Byte {
+    /// Converts this byte to a `char` if it's ASCII. Otherwise panics.
+    fn to_char(self) -> char;
+}
+
+#[cfg(test)]
+impl Byte for u8 {
+    fn to_char(self) -> char {
+        assert!(self.is_ascii());
+        char::from(self)
+    }
+}