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>

.github/workflows/base-image.yml

@@ -0,0 +1,77 @@
+name: Build Base Image (Stages 0-2)
+
+on:
+  schedule:
+    # Weekly on Sunday at 04:00 UTC
+    - cron: '0 4 * * 0'
+  workflow_dispatch:
+
+permissions:
+  contents: write
+
+jobs:
+  build-base:
+    runs-on: ubuntu-24.04-arm64
+    timeout-minutes: 120
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up pi-gen
+        run: |
+          git clone --depth 1 https://github.com/RPi-Distro/pi-gen.git /tmp/pi-gen-builder
+
+      - name: Configure base-only build
+        run: |
+          cd /tmp/pi-gen-builder
+
+          # Copy our config but override STAGE_LIST to only build base stages
+          cp "${GITHUB_WORKSPACE}/pi-gen/config" config
+          sed -i 's/^STAGE_LIST=.*/STAGE_LIST="stage0 stage1 stage2"/' config
+
+          # Mark stage2 for export so we get the rootfs
+          touch stage2/EXPORT_IMAGE
+
+      - name: Build base image
+        run: |
+          cd /tmp/pi-gen-builder
+          PIGEN_DOCKER_MODE=1 ./build-docker.sh
+
+      - name: Package base rootfs
+        run: |
+          cd /tmp/pi-gen-builder
+          DATE="$(date -u +%Y%m%d)"
+
+          # Find the stage2 rootfs in the work directory
+          ROOTFS_DIR=$(find work/ -path "*/stage2/rootfs" -type d | head -1)
+          if [ -z "$ROOTFS_DIR" ]; then
+            echo "ERROR: stage2 rootfs not found"
+            exit 1
+          fi
+
+          echo "Packaging rootfs from: $ROOTFS_DIR"
+          tar -czf "${GITHUB_WORKSPACE}/base-image-trixie-arm64-${DATE}.tar.gz" \
+            -C "$(dirname "$ROOTFS_DIR")" rootfs/
+
+          ls -lh "${GITHUB_WORKSPACE}/base-image-trixie-arm64-${DATE}.tar.gz"
+
+      - name: Create base image release
+        env:
+          GH_TOKEN: ${{ github.token }}
+        run: |
+          DATE="$(date -u +%Y%m%d)"
+          TAG="base-v${DATE}"
+          TARBALL="base-image-trixie-arm64-${DATE}.tar.gz"
+
+          # Delete previous base release if exists (keep only latest)
+          gh release delete "$TAG" --yes 2>/dev/null || true
+          git push --delete origin "$TAG" 2>/dev/null || true
+
+          # Create new release
+          gh release create "$TAG" "$TARBALL" \
+            --title "Base Image ${DATE}" \
+            --notes "Pre-baked base image (stages 0-2) for Debian Trixie arm64.
+
+          Use with: BASE_IMAGE=./${TARBALL} ./build-image.sh full
+
+          Built: $(date -u +%Y-%m-%d\ %H:%M:%S\ UTC)" \
+            --prerelease