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_actions.sh

@@ -0,0 +1,51 @@
+#!/usr/bin/env bash
+#
+# 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.
+
+set -eo pipefail
+
+script_name="ci/check_actions.sh"
+
+# Ensure action-validator is installed
+if [ ! -x "$HOME/.cargo/bin/action-validator" ]; then
+    echo "$script_name: action-validator not found, installing..." >&2
+    # Install specific version to ensure reproducibility
+    cargo install -q action-validator --version 0.8.0 --locked
+fi
+export PATH="$HOME/.cargo/bin:$PATH"
+
+# Files to exclude from validation (e.g., because they are not Actions/Workflows)
+# Use relative paths matching `find .github` output
+EXCLUDE_FILES=(
+    "./.github/dependabot.yml"
+    "./.github/release.yml"
+)
+
+failed=0
+
+# Use process substitution and while loop to handle filenames with spaces robustly
+while IFS= read -r -d '' file; do
+    # Check if file is in exclusion list
+    for exclude in "${EXCLUDE_FILES[@]}"; do
+        if [[ "$file" == "$exclude" ]]; then
+            continue 2
+        fi
+    done
+
+    if ! output=$(action-validator "$file" 2>&1); then
+        echo "$script_name: ❌ Validation failed for $file" >&2
+        echo "$output" | sed "s|^|$script_name:   |" >&2
+        failed=1
+    fi
+done < <(find ./.github -type f \( -iname '*.yml' -o -iname '*.yaml' \) -print0)
+
+if [[ $failed -ne 0 ]]; then
+    echo "$script_name: One or more files failed validation." >&2
+    exit 1
+fi