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/priv_in_pub.rs

@@ -0,0 +1,36 @@
+// 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");
+
+// FIXME(#847): Make this test succeed on earlier Rust versions.
+#[::rustversion::stable(1.59)]
+mod test {
+    use super::*;
+
+    // These derives do not result in E0446 as of Rust 1.59.0, because of
+    // https://github.com/rust-lang/rust/pull/90586.
+    //
+    // This change eliminates one of the major downsides of emitting `where`
+    // bounds for field types (i.e., the emission of E0446 for private field
+    // types).
+
+    #[derive(imp::KnownLayout, imp::IntoBytes, imp::FromZeros, imp::FromBytes, imp::Unaligned)]
+    #[zerocopy(crate = "zerocopy_renamed")]
+    #[repr(C)]
+    pub struct Public(Private);
+
+    #[derive(imp::KnownLayout, imp::IntoBytes, imp::FromZeros, imp::FromBytes, imp::Unaligned)]
+    #[zerocopy(crate = "zerocopy_renamed")]
+    #[repr(C)]
+    struct Private(());
+}