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/zerocopy/tests/include.rs

@@ -0,0 +1,67 @@
+// Copyright 2026 The Fuchsia Authors
+//
+// Licensed under a BSD-style license <LICENSE-BSD>, Apache License, Version 2.0
+// <LICENSE-APACHE or https://www.apache.org/licenses/LICENSE-2.0>, or the MIT
+// license <LICENSE-MIT or https://opensource.org/licenses/MIT>, at your option.
+// This file may not be copied, modified, or distributed except according to
+// those terms.
+
+extern crate zerocopy;
+
+#[allow(unused)]
+#[cfg(feature = "derive")]
+mod util {
+    /// A type that doesn't implement any zerocopy traits.
+    pub struct NotZerocopy<T = ()>(pub T);
+
+    /// A `u16` with alignment 2.
+    ///
+    /// Though `u16` has alignment 2 on some platforms, it's not guaranteed. By
+    /// contrast, `util::AU16` is guaranteed to have alignment 2.
+    #[derive(
+        zerocopy::KnownLayout,
+        zerocopy::Immutable,
+        zerocopy::FromBytes,
+        zerocopy::IntoBytes,
+        Copy,
+        Clone,
+    )]
+    #[repr(C, align(2))]
+    pub struct AU16(pub u16);
+
+    // Since we can't import these by path (ie, `util::assert_impl_all!`), use a
+    // name prefix to ensure our derive-emitted code isn't accidentally relying
+    // on `assert_impl_all!` being in scope.
+    #[macro_export]
+    macro_rules! util_assert_impl_all {
+        ($type:ty: $($trait:path),+ $(,)?) => {
+            const _: fn() = || {
+                use ::core::prelude::v1::*;
+                ::static_assertions::assert_impl_all!($type: $($trait),+);
+            };
+        };
+    }
+
+    // Since we can't import these by path (ie, `util::assert_not_impl_any!`),
+    // use a name prefix to ensure our derive-emitted code isn't accidentally
+    // relying on `assert_not_impl_any!` being in scope.
+    #[macro_export]
+    macro_rules! util_assert_not_impl_any {
+        ($x:ty: $($t:path),+ $(,)?) => {
+            const _: fn() = || {
+                use ::core::prelude::v1::*;
+                ::static_assertions::assert_not_impl_any!($x: $($t),+);
+            };
+        };
+    }
+
+    #[macro_export]
+    macro_rules! test_trivial_is_bit_valid {
+        ($x:ty => $name:ident) => {
+            #[test]
+            fn $name() {
+                util::test_trivial_is_bit_valid::<$x>();
+            }
+        };
+    }
+}