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/CHANGELOG.md

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+<!-- 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. -->
+
+# Changelog
+
+## Releases
+
+We track releases and release notes using [GitHub
+Releases](https://github.com/google/zerocopy/releases).
+
+## Yanks and Regressions
+
+### 0.2.2 through 0.2.8, 0.3.0 through 0.3.1, 0.4.0, 0.5.0, 0.6.0 through 0.6.5, 0.7.0 through 0.7.30
+
+*Security advisories for this bug have been published as
+[RUSTSEC-2023-0074][rustsec-advisory] and [GHSA-3mv5-343c-w2qg][github-advisory].*
+
+In these versions, the `Ref` methods `into_ref`, `into_mut`, `into_slice`, and
+`into_mut_slice` were permitted in combination with the standard library
+`cell::Ref` and `cell::RefMut` types for `Ref<B, T>`'s `B` type parameter. These
+combinations are unsound, and may permit safe code to exhibit undefined
+behavior. Fixes have been published to each affected minor version which do not
+permit this code to compile.
+
+See [#716][issue-716] for more details.
+
+[rustsec-advisory]: https://rustsec.org/advisories/RUSTSEC-2023-0074.html
+[github-advisory]: https://github.com/google/zerocopy/security/advisories/GHSA-3mv5-343c-w2qg
+[issue-716]: https://github.com/google/zerocopy/issues/716
+
+### 0.7.27, 0.7.28
+
+These versions were briefly yanked due to a non-soundness regression reported in
+[#672][pull-672]. After reconsidering our yanking policy in [#679][issue-679],
+we un-yanked these versions.
+
+[pull-672]: https://github.com/google/zerocopy/pull/672
+[issue-679]: https://github.com/google/zerocopy/issues/679