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/style.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. -->
+
+# Style Guidelines
+
+This document covers code style and formatting guidelines for the project, as
+well as commit message requirements.
+
+## File Headers
+
+Each file must contain a copyright header (see `src/lib.rs` for example) which is
+based on that file's creation year.
+
+## Formatting
+
+Refer to `ci/check_fmt.sh`.
+
+## Comments
+
+- Wrap all comments (`//`, `///`, `//!`) at **80 columns** from the left margin,
+  taking into account any preceding code or comments.
+- **Exceptions:** Markdown tables, ASCII diagrams, long URLs, code blocks, or
+  other cases where wrapping would impair readability.
+
+## Markdown Files
+
+- Wrap paragraphs and bulleted lists at **80 columns** from the left margin,
+  taking into account any preceding code or comments. For example, a markdown
+  block inside of a `/// Lorem ipsum...` comment should have lines no more than
+  76 columns wide.
+  - In bulleted lists, indent subsequent lines by 2 spaces.
+  - Do not wrap links if it breaks them.
+- Always put a blank line between a section header and the beginning of the section.
+
+## Pull Requests and Commit Messages
+
+Use GitHub issue syntax in commit messages:
+
+- Resolves issue: `Closes #123`
+- Progress on issue: `Makes progress on #123`