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/ci/check_all_toolchains_tested.sh

@@ -0,0 +1,25 @@
+#!/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
+
+# Check whether the set of toolchains tested in this file (other than
+# 'msrv', 'stable', and 'nightly') is equal to the set of toolchains
+# listed in the 'package.metadata.build-rs' section of Cargo.toml.
+#
+# If the inputs to `diff` are not identical, `diff` exits with a
+# non-zero error code, which causes this script to fail (thanks to
+# `set -e`).
+diff \
+  <(yq -r '.jobs.build_test.strategy.matrix.toolchain | .[]' .github/workflows/ci.yml | \
+    sort -u | grep -v '^\(msrv\|stable\|nightly\)$') \
+  <(cargo metadata -q --format-version 1 | \
+    jq -r ".packages[] | select(.name == \"zerocopy\").metadata.\"build-rs\" | keys | .[]" | \
+    sort -u) >&2