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/unsafe_cell.rs

@@ -0,0 +1,36 @@
+// Copyright 2025 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");
+
+// Test to make sure that all of our derives are compatible with `UnsafeCell`s.
+//
+// We test both `FromBytes` and `FromZeros`, as the `FromBytes` implied derive
+// of `TryFromBytes` emits a trivial `is_bit_valid` impl - we want to test the
+// non-trivial impl, which deriving `FromZeros` accomplishes.
+
+#[derive(imp::FromBytes, imp::IntoBytes, imp::KnownLayout, imp::Unaligned)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C)]
+struct StructFromBytes(imp::UnsafeCell<u8>);
+
+#[derive(imp::FromZeros, imp::IntoBytes, imp::KnownLayout, imp::Unaligned)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C)]
+struct StructFromZeros(imp::UnsafeCell<bool>);
+
+#[derive(imp::FromZeros, imp::IntoBytes, imp::KnownLayout, imp::Unaligned)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(u8)]
+enum EnumFromZeros {
+    A(imp::UnsafeCell<bool>),
+}