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/hashbrown/src/scopeguard.rs

@@ -0,0 +1,72 @@
+// Extracted from the scopeguard crate
+use core::{
+    mem::ManuallyDrop,
+    ops::{Deref, DerefMut},
+    ptr,
+};
+
+pub struct ScopeGuard<T, F>
+where
+    F: FnMut(&mut T),
+{
+    dropfn: F,
+    value: T,
+}
+
+#[inline]
+pub fn guard<T, F>(value: T, dropfn: F) -> ScopeGuard<T, F>
+where
+    F: FnMut(&mut T),
+{
+    ScopeGuard { dropfn, value }
+}
+
+impl<T, F> ScopeGuard<T, F>
+where
+    F: FnMut(&mut T),
+{
+    #[inline]
+    pub fn into_inner(guard: Self) -> T {
+        // Cannot move out of Drop-implementing types, so
+        // ptr::read the value out of a ManuallyDrop<Self>
+        // Don't use mem::forget as that might invalidate value
+        let guard = ManuallyDrop::new(guard);
+        unsafe {
+            let value = ptr::read(&guard.value);
+            // read the closure so that it is dropped
+            let _ = ptr::read(&guard.dropfn);
+            value
+        }
+    }
+}
+
+impl<T, F> Deref for ScopeGuard<T, F>
+where
+    F: FnMut(&mut T),
+{
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &T {
+        &self.value
+    }
+}
+
+impl<T, F> DerefMut for ScopeGuard<T, F>
+where
+    F: FnMut(&mut T),
+{
+    #[inline]
+    fn deref_mut(&mut self) -> &mut T {
+        &mut self.value
+    }
+}
+
+impl<T, F> Drop for ScopeGuard<T, F>
+where
+    F: FnMut(&mut T),
+{
+    #[inline]
+    fn drop(&mut self) {
+        (self.dropfn)(&mut self.value);
+    }
+}