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/fixed/src/borshize.rs

@@ -0,0 +1,50 @@
+// Copyright © 2018–2025 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::{
+    FixedI8, FixedI16, FixedI32, FixedI64, FixedI128, FixedU8, FixedU16, FixedU32, FixedU64,
+    FixedU128,
+};
+use borsh::io::{Read, Result, Write};
+use borsh::{BorshDeserialize, BorshSerialize};
+
+macro_rules! borsh_fixed {
+    ($Fixed:ident is $TBits:ident) => {
+        impl<Frac> BorshSerialize for $Fixed<Frac> {
+            #[inline]
+            fn serialize<W: Write>(&self, writer: &mut W) -> Result<()> {
+                <$TBits as BorshSerialize>::serialize(&self.bits, writer)
+            }
+        }
+
+        impl<Frac> BorshDeserialize for $Fixed<Frac> {
+            #[inline]
+            fn deserialize_reader<R: Read>(reader: &mut R) -> Result<Self> {
+                <$TBits as BorshDeserialize>::deserialize_reader(reader).map($Fixed::from_bits)
+            }
+        }
+    };
+}
+
+borsh_fixed! { FixedI8 is i8 }
+borsh_fixed! { FixedI16 is i16 }
+borsh_fixed! { FixedI32 is i32 }
+borsh_fixed! { FixedI64 is i64 }
+borsh_fixed! { FixedI128 is i128 }
+borsh_fixed! { FixedU8 is u8 }
+borsh_fixed! { FixedU16 is u16 }
+borsh_fixed! { FixedU32 is u32 }
+borsh_fixed! { FixedU64 is u64 }
+borsh_fixed! { FixedU128 is u128 }