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/agent_docs/development.md

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+<!-- 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. -->
+
+# Development Guidelines
+
+This document covers guidelines for developing code changes.
+
+## Build and Test
+
+This repository uses a wrapper script (`cargo.sh`) to ensure consistent
+toolchain usage and configuration.
+
+> [!IMPORTANT]
+> **NEVER** run `cargo` directly.
+> **ALWAYS** use `./cargo.sh` for all cargo sub-commands.
+>
+> **Why?** `cargo.sh` ensures that the toolchains used in development match
+> those in CI, which is important because some features are only available on
+> specific toolchains, and because UI tests rely on the text of compiler errors,
+> which changes between toolchain versions.
+
+### Syntax
+
+`./cargo.sh +<toolchain> <command> [args]`
+
+This is equivalent to:
+
+`cargo +1.2.3 <command> [args]`
+
+...where `1.2.3` is the toolchain version named by `<toolchain>` (e.g., `msrv` ->
+`1.56.0`).
+
+### Toolchains
+
+The `<toolchain>` argument is mandatory:
+- `msrv`: Minimum Supported Rust Version.
+- `stable`: Stable toolchain.
+- `nightly`: Nightly toolchain.
+- `all`: Runs on `msrv`, `stable`, and `nightly` sequentially.
+- Version-gated: e.g., `no-zerocopy-core-error-1-81-0` (see `Cargo.toml`).
+
+
+## MSRV (Minimum Supported Rust Version)
+
+The MSRV is **1.56.0**.
+
+- **Do NOT** use features stabilized after 1.56.0 unless version-gated.
+- **Requirement:** Ask for user approval before introducing new version-gated
+  behavior.
+- **Verify**: Ensure code compiles on 1.56.0 (`./cargo.sh +msrv ...`).
+
+### Version Gating Convention
+
+We use `[package.metadata.build-rs]` in `Cargo.toml` to gate features by Rust version.
+
+1.  **Define**: Add `no-zerocopy-<feature>-<version> = "<version>"` to `Cargo.toml`.
+2.  **Use**: Use `#[cfg(not(no_zerocopy_<feature>_<version>))]` (note underscores).
+3.  **Document**: For public items, use `#[cfg_attr(doc_cfg, doc(cfg(rust = "<version>")))]`.
+
+**Important:** The toolchains listed in `.github/workflows/ci.yml` and
+`Cargo.toml` (under `[package.metadata.build-rs]`) must be kept in sync. If you
+add a new version-gated feature, ensure it is reflected in both places.
+
+## UI Tests
+
+For advice on how to add, modify, or remove UI tests (in `tests/ui-*` or
+`zerocopy-derive/tests/ui-*`), refer to [agent_docs/ui_tests.md](./ui_tests.md).
+
+## Macro Development
+
+- **Shared Logic:** Put shared macro logic in `src/util/macro_util.rs` to avoid
+  code duplication in generated code.
+- **Lints:** Generated code often triggers lints. Use `#[allow(...)]` liberally
+  in generated code to suppress them.
+  ```rust
+  // GOOD: Suppress lints that might be triggered by generated names.
+  // Example: Using a variant name (PascalCase) as a field name (snake_case).
+  // Input: `enum MyEnum { VariantA }`
+  // Generated: `union Variants { __field_VariantA: ... }`
+  quote! {
+      #[allow(non_snake_case)]
+      union ___ZerocopyVariants {
+          #(#fields)*
+      }
+  }
+  ```
+
+## Unsafe Code
+
+`unsafe` code is extremely dangerous and should be avoided unless absolutely
+necessary. For guidelines on writing unsafe code, including pointer casts and
+safety comments, refer to [agent_docs/unsafe_code.md](./unsafe_code.md).