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_to_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::IntoBytes` if:
+// - all fields are `imp::IntoBytes`
+// - `repr(C)` or `repr(transparent)` and
+//   - no padding (size of union equals size of each field type)
+// - `repr(packed)`
+
+#[derive(imp::IntoBytes, Clone, Copy)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C)]
+union CZst {
+    a: (),
+}
+
+util_assert_impl_all!(CZst: imp::IntoBytes);
+
+#[derive(imp::IntoBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C)]
+union C {
+    a: u8,
+    b: u8,
+}
+
+util_assert_impl_all!(C: imp::IntoBytes);
+
+#[derive(imp::IntoBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C, align(2))]
+union Aligned {
+    a: [u8; 2],
+}
+
+util_assert_impl_all!(Aligned: imp::IntoBytes);
+
+// Transparent unions are unstable; see issue #60405
+// <https://github.com/rust-lang/rust/issues/60405> for more information.
+
+// #[derive(imp::IntoBytes)]
+// #[repr(transparent)]
+// union Transparent {
+//     a: u8,
+//     b: CZst,
+// }
+
+// is_as_bytes!(Transparent);
+
+#[derive(imp::IntoBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C, packed)]
+union CZstPacked {
+    a: (),
+}
+
+util_assert_impl_all!(CZstPacked: imp::IntoBytes);
+
+#[derive(imp::IntoBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C, packed)]
+union CPacked {
+    a: u8,
+    b: i8,
+}
+
+util_assert_impl_all!(CPacked: imp::IntoBytes);
+
+#[derive(imp::IntoBytes)]
+#[zerocopy(crate = "zerocopy_renamed")]
+#[repr(C, packed)]
+union CMultibytePacked {
+    a: i32,
+    b: u32,
+    c: f32,
+}
+
+util_assert_impl_all!(CMultibytePacked: imp::IntoBytes);