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/ui_tests.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. -->
+
+# UI & Output Tests
+
+When updating UI test files (`tests/ui-*` or `zerocopy-derive/tests/ui-*`) or
+functionality which could affect compiler error output or derive output, run:
+`./tools/update-expected-test-output.sh`.
+
+**Note:** We maintain separate UI tests for different toolchains (`ui-msrv`,
+`ui-stable`, `ui-nightly`) because compiler output varies. The script handles
+this automatically.
+
+### Symlink Pattern
+
+To share test code across toolchains while allowing for different error output,
+we use a symlink pattern:
+
+1.  **Canonical Source:** The `ui-nightly` directory holds the actual source
+    files (`.rs`).
+2.  **Symlinks:** The `ui-stable` and `ui-msrv` directories contain *symlinks*
+    to the `.rs` files in `ui-nightly`.
+    - **Example:** `tests/ui-stable/foo.rs` -> `../ui-nightly/foo.rs`
+3.  **Unique Output:** Each directory contains its own `.stderr` files.
+
+### Workflow Rules
+
+- **Adding a Test:**
+    1.  Create the `.rs` file in `ui-nightly`.
+    2.  Create relative symlinks in `ui-stable` and `ui-msrv` pointing to the
+        new file in `ui-nightly`.
+    3.  Run `./tools/update-expected-test-output.sh` to generate the `.stderr` files.
+- **Modifying a Test:**
+    1.  Edit the `.rs` file in `ui-nightly`.
+    2.  Run `./tools/update-expected-test-output.sh` to update the `.stderr` files.
+- **Removing a Test:**
+    1.  Delete the `.rs` file from `ui-nightly`.
+    2.  Delete the symlinks from `ui-stable` and `ui-msrv`.
+    3.  Delete the corresponding `.stderr` files from all three directories.
+
+**NEVER** edit `.stderr` files directly. Only update them via the script or the
+commands it runs. If a `.stderr` file is causing a test failure and updating it
+via tooling does not fix the failure, that indicates a bug.