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>
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