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/enum_known_layout.rs

@@ -0,0 +1,54 @@
+// 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::KnownLayout)]
+#[zerocopy(crate = "zerocopy_renamed")]
+enum Foo {
+    A,
+}
+
+util_assert_impl_all!(Foo: imp::KnownLayout);
+
+#[derive(imp::KnownLayout)]
+#[zerocopy(crate = "zerocopy_renamed")]
+enum Bar {
+    A = 0,
+}
+
+util_assert_impl_all!(Bar: imp::KnownLayout);
+
+#[derive(imp::KnownLayout)]
+#[zerocopy(crate = "zerocopy_renamed")]
+enum Baz {
+    A = 1,
+    B = 0,
+}
+
+util_assert_impl_all!(Baz: imp::KnownLayout);
+
+// Deriving `KnownLayout` should work if the enum has bounded parameters.
+
+#[derive(imp::KnownLayout)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C)]
+enum WithParams<'a: 'b, 'b: 'a, T: 'a + 'b + imp::KnownLayout, const N: usize>
+where
+    'a: 'b,
+    'b: 'a,
+    T: 'a + 'b + imp::KnownLayout,
+{
+    Variant([T; N], imp::PhantomData<&'a &'b ()>),
+}
+
+util_assert_impl_all!(WithParams<'static, 'static, u8, 42>: imp::KnownLayout);