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-0.13.2/src/raw/alloc.rs

@@ -0,0 +1,73 @@
+pub(crate) use self::inner::{do_alloc, Allocator, Global};
+
+#[cfg(feature = "nightly")]
+mod inner {
+    use crate::alloc::alloc::Layout;
+    pub use crate::alloc::alloc::{Allocator, Global};
+    use core::ptr::NonNull;
+
+    #[allow(clippy::map_err_ignore)]
+    pub fn do_alloc<A: Allocator>(alloc: &A, layout: Layout) -> Result<NonNull<u8>, ()> {
+        match alloc.allocate(layout) {
+            Ok(ptr) => Ok(ptr.as_non_null_ptr()),
+            Err(_) => Err(()),
+        }
+    }
+
+    #[cfg(feature = "bumpalo")]
+    unsafe impl Allocator for crate::BumpWrapper<'_> {
+        #[inline]
+        fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, core::alloc::AllocError> {
+            match self.0.try_alloc_layout(layout) {
+                Ok(ptr) => Ok(NonNull::slice_from_raw_parts(ptr, layout.size())),
+                Err(_) => Err(core::alloc::AllocError),
+            }
+        }
+        #[inline]
+        unsafe fn deallocate(&self, _ptr: NonNull<u8>, _layout: Layout) {}
+    }
+}
+
+#[cfg(not(feature = "nightly"))]
+mod inner {
+    use crate::alloc::alloc::{alloc, dealloc, Layout};
+    use core::ptr::NonNull;
+
+    #[allow(clippy::missing_safety_doc)] // not exposed outside of this crate
+    pub unsafe trait Allocator {
+        fn allocate(&self, layout: Layout) -> Result<NonNull<u8>, ()>;
+        unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout);
+    }
+
+    #[derive(Copy, Clone)]
+    pub struct Global;
+    unsafe impl Allocator for Global {
+        #[inline]
+        fn allocate(&self, layout: Layout) -> Result<NonNull<u8>, ()> {
+            unsafe { NonNull::new(alloc(layout)).ok_or(()) }
+        }
+        #[inline]
+        unsafe fn deallocate(&self, ptr: NonNull<u8>, layout: Layout) {
+            dealloc(ptr.as_ptr(), layout);
+        }
+    }
+    impl Default for Global {
+        #[inline]
+        fn default() -> Self {
+            Global
+        }
+    }
+
+    pub fn do_alloc<A: Allocator>(alloc: &A, layout: Layout) -> Result<NonNull<u8>, ()> {
+        alloc.allocate(layout)
+    }
+
+    #[cfg(feature = "bumpalo")]
+    unsafe impl Allocator for crate::BumpWrapper<'_> {
+        #[allow(clippy::map_err_ignore)]
+        fn allocate(&self, layout: Layout) -> Result<NonNull<u8>, ()> {
+            self.0.try_alloc_layout(layout).map_err(|_| ())
+        }
+        unsafe fn deallocate(&self, _ptr: NonNull<u8>, _layout: Layout) {}
+    }
+}