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-derive/tests/union_from_bytes.rs

@@ -0,0 +1,89 @@
+// Copyright 2019 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.
+
+// See comment in `include.rs` for why we disable the prelude.
+#![no_implicit_prelude]
+#![allow(warnings)]
+
+include!("include.rs");
+
+// A union is `imp::FromBytes` if:
+// - all fields are `imp::FromBytes`
+
+#[derive(Clone, Copy, imp::Immutable, imp::FromBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+union Zst {
+    a: (),
+}
+
+util_assert_impl_all!(Zst: imp::FromBytes);
+test_trivial_is_bit_valid!(Zst => test_zst_trivial_is_bit_valid);
+
+#[derive(imp::Immutable, imp::FromBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+union One {
+    a: u8,
+}
+
+util_assert_impl_all!(One: imp::FromBytes);
+test_trivial_is_bit_valid!(One => test_one_trivial_is_bit_valid);
+
+#[derive(imp::Immutable, imp::FromBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+union Two {
+    a: u8,
+    b: Zst,
+}
+
+util_assert_impl_all!(Two: imp::FromBytes);
+test_trivial_is_bit_valid!(Two => test_two_trivial_is_bit_valid);
+
+#[derive(imp::Immutable, imp::FromBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+union TypeParams<'a, T: imp::Copy, I: imp::Iterator>
+where
+    I::Item: imp::Copy,
+{
+    a: T,
+    c: I::Item,
+    d: u8,
+    e: imp::PhantomData<&'a [u8]>,
+    f: imp::PhantomData<&'static str>,
+    g: imp::PhantomData<imp::String>,
+}
+
+util_assert_impl_all!(TypeParams<'static, (), imp::IntoIter<()>>: imp::FromBytes);
+test_trivial_is_bit_valid!(TypeParams<'static, (), imp::IntoIter<()>> => test_type_params_trivial_is_bit_valid);
+
+// Deriving `imp::FromBytes` should work if the union has bounded parameters.
+
+#[derive(imp::Immutable, imp::FromBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C)]
+union WithParams<'a: 'b, 'b: 'a, T: 'a + 'b + imp::FromBytes, const N: usize>
+where
+    'a: 'b,
+    'b: 'a,
+    T: 'a + 'b + imp::Copy + imp::FromBytes,
+{
+    a: [T; N],
+    b: imp::PhantomData<&'a &'b ()>,
+}
+
+util_assert_impl_all!(WithParams<'static, 'static, u8, 42>: imp::FromBytes);
+test_trivial_is_bit_valid!(WithParams<'static, 'static, u8, 42> => test_with_params_trivial_is_bit_valid);
+
+#[derive(imp::FromBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C)]
+union UnsafeCellUnion {
+    a: imp::ManuallyDrop<imp::UnsafeCell<u8>>,
+}
+
+util_assert_impl_all!(UnsafeCellUnion: imp::FromBytes);
+test_trivial_is_bit_valid!(UnsafeCellUnion => test_unsafe_cell_union_trivial_is_bit_valid);