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/vcell/src/lib.rs

@@ -0,0 +1,52 @@
+//! Just like [`Cell`] but with [volatile] read / write operations
+//!
+//! [`Cell`]: https://doc.rust-lang.org/std/cell/struct.Cell.html
+//! [volatile]: https://doc.rust-lang.org/std/ptr/fn.read_volatile.html
+
+#![deny(missing_docs)]
+#![deny(warnings)]
+#![no_std]
+
+use core::cell::UnsafeCell;
+use core::ptr;
+
+/// Just like [`Cell`] but with [volatile] read / write operations
+///
+/// [`Cell`]: https://doc.rust-lang.org/std/cell/struct.Cell.html
+/// [volatile]: https://doc.rust-lang.org/std/ptr/fn.read_volatile.html
+#[repr(transparent)]
+pub struct VolatileCell<T> {
+    value: UnsafeCell<T>,
+}
+
+impl<T> VolatileCell<T> {
+    /// Creates a new `VolatileCell` containing the given value
+    pub const fn new(value: T) -> Self {
+        VolatileCell { value: UnsafeCell::new(value) }
+    }
+
+    /// Returns a copy of the contained value
+    #[inline(always)]
+    pub fn get(&self) -> T
+        where T: Copy
+    {
+        unsafe { ptr::read_volatile(self.value.get()) }
+    }
+
+    /// Sets the contained value
+    #[inline(always)]
+    pub fn set(&self, value: T)
+        where T: Copy
+    {
+        unsafe { ptr::write_volatile(self.value.get(), value) }
+    }
+
+    /// Returns a raw pointer to the underlying data in the cell
+    #[inline(always)]
+    pub fn as_ptr(&self) -> *mut T {
+        self.value.get()
+    }
+}
+
+// NOTE implicit because of `UnsafeCell`
+// unsafe impl<T> !Sync for VolatileCell<T> {}