Build optimization: pre-built PM3 binaries, ARM64 CI, base image caching

Replace PM3 compile-from-source in pi-gen with pre-built tarball extraction
(saves 43-58 min). Merge stagePM3 into stageDangerousPi as 02-pm3-install
substage, renumber all subsequent substages. Switch CI PM3 build to native
ARM64 runner (ubuntu-24.04-arm64) eliminating QEMU overhead. Add weekly
base-image workflow for pre-baking stages 0-2. Support PM3_TARBALL,
BASE_IMAGE, and APT_PROXY env vars in build-image.sh.

Also includes prior Phase 5 work: theme system, design system integration,
component update system, OS updates, CI build pipeline, and test results.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ci/build-backend.sh

@@ -0,0 +1,42 @@
+#!/usr/bin/env bash
+# Package the backend Python source for distribution.
+#
+# Environment variables:
+#   OUTPUT_DIR  - where to write the tarball (default: ./dist)
+#   VERSION     - version string (default: read from VERSION file)
+
+set -euo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+WORKSPACE="${SCRIPT_DIR}/.."
+OUTPUT_DIR="${OUTPUT_DIR:-${WORKSPACE}/dist}"
+VERSION="${VERSION:-$(cat "${WORKSPACE}/VERSION" 2>/dev/null || echo "0.1.0")}"
+
+echo "=== Dangerous Pi Backend Package ==="
+echo "Version: ${VERSION}"
+
+STAGING="/tmp/backend-staging/backend"
+rm -rf /tmp/backend-staging
+mkdir -p "$STAGING"
+
+# Copy backend source
+cp -r "${WORKSPACE}/app/backend" "${STAGING}/app/backend"
+
+# Copy requirements
+cp "${WORKSPACE}/requirements.txt" "${STAGING}/"
+
+cat > "${STAGING}/COMPONENT_VERSION" << EOF
+{
+  "component_id": "backend",
+  "version": "${VERSION}",
+  "build_date": "$(date -u +"%Y-%m-%dT%H:%M:%SZ")"
+}
+EOF
+
+mkdir -p "$OUTPUT_DIR"
+TARBALL="backend-${VERSION}.tar.gz"
+cd /tmp/backend-staging
+tar -czf "${OUTPUT_DIR}/${TARBALL}" backend/
+echo "✅ Built: ${OUTPUT_DIR}/${TARBALL}"
+
+rm -rf /tmp/backend-staging