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/struct_no_cell.rs

@@ -0,0 +1,105 @@
+// Copyright 2023 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");
+
+#[derive(imp::Immutable)]
+#[zerocopy(crate = "zerocopy_renamed")]
+struct Zst;
+
+util_assert_impl_all!(Zst: imp::Immutable);
+
+#[derive(imp::Immutable)]
+#[zerocopy(crate = "zerocopy_renamed")]
+struct One {
+    a: bool,
+}
+
+util_assert_impl_all!(One: imp::Immutable);
+
+#[derive(imp::Immutable)]
+#[zerocopy(crate = "zerocopy_renamed")]
+struct Two {
+    a: bool,
+    b: Zst,
+}
+
+util_assert_impl_all!(Two: imp::Immutable);
+
+#[derive(imp::Immutable)]
+#[zerocopy(crate = "zerocopy_renamed")]
+struct Three {
+    a: [u8],
+}
+
+util_assert_impl_all!(Three: imp::Immutable);
+
+#[derive(imp::Immutable)]
+#[zerocopy(crate = "zerocopy_renamed")]
+struct Four<'a> {
+    field: &'a imp::UnsafeCell<u8>,
+}
+
+util_assert_impl_all!(Four<'static>: imp::Immutable);
+
+#[derive(imp::Immutable)]
+#[zerocopy(crate = "zerocopy_renamed")]
+struct TypeParams<'a, T, U, I: imp::Iterator> {
+    a: I::Item,
+    b: u8,
+    c: imp::PhantomData<&'a [::core::primitive::u8]>,
+    d: imp::PhantomData<&'static ::core::primitive::str>,
+    e: imp::PhantomData<imp::String>,
+    f: imp::PhantomData<U>,
+    g: T,
+}
+
+util_assert_impl_all!(TypeParams<'static, (), (), imp::IntoIter<()>>: imp::Immutable);
+util_assert_impl_all!(TypeParams<'static, util::AU16, util::AU16, imp::IntoIter<()>>: imp::Immutable);
+util_assert_impl_all!(TypeParams<'static, util::AU16, imp::UnsafeCell<u8>, imp::IntoIter<()>>: imp::Immutable);
+util_assert_not_impl_any!(TypeParams<'static, imp::UnsafeCell<()>, (), imp::IntoIter<()>>: imp::Immutable);
+util_assert_not_impl_any!(TypeParams<'static, [imp::UnsafeCell<u8>; 0], (), imp::IntoIter<()>>: imp::Immutable);
+util_assert_not_impl_any!(TypeParams<'static, (), (), imp::IntoIter<imp::UnsafeCell<()>>>: imp::Immutable);
+
+trait Trait {
+    type Assoc;
+}
+
+impl<T> Trait for imp::UnsafeCell<T> {
+    type Assoc = T;
+}
+
+#[derive(imp::Immutable)]
+#[zerocopy(crate = "zerocopy_renamed")]
+struct WithAssocType<T: Trait> {
+    field: <T as Trait>::Assoc,
+}
+
+util_assert_impl_all!(WithAssocType<imp::UnsafeCell<u8>>: imp::Immutable);
+
+// Deriving `Immutable` should work if the struct has bounded parameters.
+
+#[derive(imp::Immutable)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C)]
+struct WithParams<'a: 'b, 'b: 'a, T: 'a + 'b + imp::Immutable, const N: usize>(
+    [T; N],
+    imp::PhantomData<&'a &'b ()>,
+    imp::PhantomData<imp::UnsafeCell<()>>,
+    &'a imp::UnsafeCell<()>,
+)
+where
+    'a: 'b,
+    'b: 'a,
+    T: 'a + 'b + imp::Immutable;
+
+util_assert_impl_all!(WithParams<'static, 'static, u8, 42>: imp::Immutable);