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/az/src/float.rs

@@ -0,0 +1,69 @@
+// Copyright © 2019–2021 Trevor Spiteri
+
+// This library is free software: you can redistribute it and/or
+// modify it under the terms of either
+//
+//   * the Apache License, Version 2.0 or
+//   * the MIT License
+//
+// at your option.
+//
+// You should have recieved copies of the Apache License and the MIT
+// License along with the library. If not, see
+// <https://www.apache.org/licenses/LICENSE-2.0> and
+// <https://opensource.org/licenses/MIT>.
+
+use crate::{cast, Cast, CheckedCast, Round, UnwrappedCast};
+use core::fmt::{Debug, Display, Formatter, LowerExp, Result as FmtResult, UpperExp};
+
+macro_rules! convert {
+    ($($Src:ty),* => $Dst:ty) => { $(
+        impl Cast<$Dst> for $Src {
+            #[inline]
+            fn cast(self) -> $Dst {
+                self as $Dst
+            }
+        }
+
+        impl CheckedCast<$Dst> for $Src {
+            #[inline]
+            fn checked_cast(self) -> Option<$Dst> {
+                Some(cast(self))
+            }
+        }
+
+        impl UnwrappedCast<$Dst> for $Src {
+            #[inline]
+            fn unwrapped_cast(self) -> $Dst {
+                cast(self)
+            }
+        }
+    )* };
+}
+
+convert! { i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize, f32, f64 => f32 }
+convert! { i8, i16, i32, i64, i128, isize, u8, u16, u32, u64, u128, usize, f32, f64 => f64 }
+
+impl<T: Display> Display for Round<T> {
+    fn fmt(&self, f: &mut Formatter) -> FmtResult {
+        Display::fmt(&self.0, f)
+    }
+}
+
+impl<T: Debug> Debug for Round<T> {
+    fn fmt(&self, f: &mut Formatter) -> FmtResult {
+        Debug::fmt(&self.0, f)
+    }
+}
+
+impl<T: LowerExp> LowerExp for Round<T> {
+    fn fmt(&self, f: &mut Formatter) -> FmtResult {
+        LowerExp::fmt(&self.0, f)
+    }
+}
+
+impl<T: UpperExp> UpperExp for Round<T> {
+    fn fmt(&self, f: &mut Formatter) -> FmtResult {
+        UpperExp::fmt(&self.0, f)
+    }
+}