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/cargo.sh

@@ -0,0 +1,17 @@
+#!/usr/bin/env bash
+#
+# Copyright 2024 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.
+
+set -eo pipefail
+
+# Build `cargo-zerocopy` without any RUSTFLAGS or CARGO_TARGET_DIR set in the
+# environment
+env -u RUSTFLAGS -u CARGO_TARGET_DIR cargo +stable build --config tools/.cargo/config.toml --manifest-path tools/Cargo.toml -p cargo-zerocopy -q
+# Thin wrapper around the `cargo-zerocopy` binary in `tools/cargo-zerocopy`
+./tools/target/debug/cargo-zerocopy $@