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/embassy-hal-internal/src/drop.rs

@@ -0,0 +1,56 @@
+//! Types for controlling when drop is invoked.
+use core::mem;
+use core::mem::MaybeUninit;
+
+/// A type to delay the drop handler invocation.
+#[must_use = "to delay the drop handler invocation to the end of the scope"]
+pub struct OnDrop<F: FnOnce()> {
+    f: MaybeUninit<F>,
+}
+
+impl<F: FnOnce()> OnDrop<F> {
+    /// Create a new instance.
+    pub fn new(f: F) -> Self {
+        Self { f: MaybeUninit::new(f) }
+    }
+
+    /// Prevent drop handler from running.
+    pub fn defuse(self) {
+        mem::forget(self)
+    }
+}
+
+impl<F: FnOnce()> Drop for OnDrop<F> {
+    fn drop(&mut self) {
+        unsafe { self.f.as_ptr().read()() }
+    }
+}
+
+/// An explosive ordinance that panics if it is improperly disposed of.
+///
+/// This is to forbid dropping futures, when there is absolutely no other choice.
+///
+/// To correctly dispose of this device, call the [defuse](struct.DropBomb.html#method.defuse)
+/// method before this object is dropped.
+#[must_use = "to delay the drop bomb invokation to the end of the scope"]
+pub struct DropBomb {
+    _private: (),
+}
+
+impl DropBomb {
+    /// Create a new instance.
+    pub fn new() -> Self {
+        Self { _private: () }
+    }
+
+    /// Defuses the bomb, rendering it safe to drop.
+    pub fn defuse(self) {
+        mem::forget(self)
+    }
+}
+
+impl Drop for DropBomb {
+    fn drop(&mut self) {
+        panic!("boom")
+    }
+}