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/ahash/src/convert.rs

@@ -0,0 +1,151 @@
+pub(crate) trait Convert<To> {
+    fn convert(self) -> To;
+}
+
+macro_rules! convert {
+    ($a:ty, $b:ty) => {
+        impl Convert<$b> for $a {
+            #[inline(always)]
+            fn convert(self) -> $b {
+                zerocopy::transmute!(self)
+            }
+        }
+        impl Convert<$a> for $b {
+            #[inline(always)]
+            fn convert(self) -> $a {
+                zerocopy::transmute!(self)
+            }
+        }
+    };
+}
+
+macro_rules! convert_primitive_bytes {
+    ($a:ty, $b:ty) => {
+        impl Convert<$b> for $a {
+            #[inline(always)]
+            fn convert(self) -> $b {
+                self.to_ne_bytes()
+            }
+        }
+        impl Convert<$a> for $b {
+            #[inline(always)]
+            fn convert(self) -> $a {
+                <$a>::from_ne_bytes(self)
+            }
+        }
+    };
+}
+
+convert!([u128; 4], [u8; 64]);
+convert!([u128; 2], [u64; 4]);
+convert!([u128; 2], [u8; 32]);
+convert!(u128, [u64; 2]);
+convert_primitive_bytes!(u128, [u8; 16]);
+convert!([u64; 2], [u32; 4]);
+#[cfg(test)]
+convert!([u64; 2], [u8; 16]);
+convert_primitive_bytes!(u64, [u8; 8]);
+convert_primitive_bytes!(u32, [u8; 4]);
+convert_primitive_bytes!(u16, [u8; 2]);
+convert!([[u64; 4]; 2], [u8; 64]);
+
+macro_rules! as_array {
+    ($input:expr, $len:expr) => {{
+        {
+            #[inline(always)]
+            fn as_array<T>(slice: &[T]) -> &[T; $len] {
+                core::convert::TryFrom::try_from(slice).unwrap()
+            }
+            as_array($input)
+        }
+    }};
+}
+
+pub(crate) trait ReadFromSlice {
+    fn read_u16(&self) -> (u16, &[u8]);
+    fn read_u32(&self) -> (u32, &[u8]);
+    fn read_u64(&self) -> (u64, &[u8]);
+    fn read_u128(&self) -> (u128, &[u8]);
+    fn read_u128x2(&self) -> ([u128; 2], &[u8]);
+    fn read_u128x4(&self) -> ([u128; 4], &[u8]);
+    fn read_last_u16(&self) -> u16;
+    fn read_last_u32(&self) -> u32;
+    fn read_last_u64(&self) -> u64;
+    fn read_last_u128(&self) -> u128;
+    fn read_last_u128x2(&self) -> [u128; 2];
+    fn read_last_u128x4(&self) -> [u128; 4];
+}
+
+impl ReadFromSlice for [u8] {
+    #[inline(always)]
+    fn read_u16(&self) -> (u16, &[u8]) {
+        let (value, rest) = self.split_at(2);
+        (as_array!(value, 2).convert(), rest)
+    }
+
+    #[inline(always)]
+    fn read_u32(&self) -> (u32, &[u8]) {
+        let (value, rest) = self.split_at(4);
+        (as_array!(value, 4).convert(), rest)
+    }
+
+    #[inline(always)]
+    fn read_u64(&self) -> (u64, &[u8]) {
+        let (value, rest) = self.split_at(8);
+        (as_array!(value, 8).convert(), rest)
+    }
+
+    #[inline(always)]
+    fn read_u128(&self) -> (u128, &[u8]) {
+        let (value, rest) = self.split_at(16);
+        (as_array!(value, 16).convert(), rest)
+    }
+
+    #[inline(always)]
+    fn read_u128x2(&self) -> ([u128; 2], &[u8]) {
+        let (value, rest) = self.split_at(32);
+        (as_array!(value, 32).convert(), rest)
+    }
+
+    #[inline(always)]
+    fn read_u128x4(&self) -> ([u128; 4], &[u8]) {
+        let (value, rest) = self.split_at(64);
+        (as_array!(value, 64).convert(), rest)
+    }
+
+    #[inline(always)]
+    fn read_last_u16(&self) -> u16 {
+        let (_, value) = self.split_at(self.len() - 2);
+        as_array!(value, 2).convert()
+    }
+
+    #[inline(always)]
+    fn read_last_u32(&self) -> u32 {
+        let (_, value) = self.split_at(self.len() - 4);
+        as_array!(value, 4).convert()
+    }
+
+    #[inline(always)]
+    fn read_last_u64(&self) -> u64 {
+        let (_, value) = self.split_at(self.len() - 8);
+        as_array!(value, 8).convert()
+    }
+
+    #[inline(always)]
+    fn read_last_u128(&self) -> u128 {
+        let (_, value) = self.split_at(self.len() - 16);
+        as_array!(value, 16).convert()
+    }
+
+    #[inline(always)]
+    fn read_last_u128x2(&self) -> [u128; 2] {
+        let (_, value) = self.split_at(self.len() - 32);
+        as_array!(value, 32).convert()
+    }
+
+    #[inline(always)]
+    fn read_last_u128x4(&self) -> [u128; 4] {
+        let (_, value) = self.split_at(self.len() - 64);
+        as_array!(value, 64).convert()
+    }
+}