.github/workflows: Migrate workflows to Blacksmith runners

[blacksmith-sh]

This PR has been automatically generated by a team member in your GitHub organization using Blacksmith's Migration Wizard. This PR changes the following:

1. Your selected workflows will now run on Blacksmith's 2x faster hardware (e.g., runs-on: blacksmith-4vcpu-ubuntu-2204). Learn more about the different instances available to choose from.
2. Your jobs running on Blacksmith will now have all official GitHub and popular third-party cache actions automatically interact with our 4x faster, colocated cache. Learn more about Blacksmith's actions cache.
3. Your GitHub Actions will now actually be observable. Learn more about Blacksmith's logging and other observability features.
4. Your Docker builds will now automatically share their Docker layer cache, resulting in up to 40x faster builds. Learn more about Blacksmith's Docker layer caching.

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Summary by cubic

Migrate GitHub Actions to Blacksmith runners and Docker builder for faster, consistent CI across Linux, macOS, and Windows; keep the macOS GUI boot-smoke job on GitHub-hosted for now. Also make setup-bun auto-install a supported Node when the runner default is too old to prevent bun install failures.

• Migration

  • Move runners: Linux to blacksmith-4vcpu-ubuntu-2404 (+ blacksmith-4vcpu-ubuntu-2404-arm), macOS to blacksmith-6vcpu-macos-15, Windows to blacksmith-4vcpu-windows-2025.
  • Keep the native app GUI boot-smoke on macos-latest until a launched-app smoke passes on Blacksmith.
  • Switch from docker/setup-buildx-action@v3/docker/build-push-action@v5 to useblacksmith/setup-docker-builder@v1/useblacksmith/build-push-action@v2 for per-arch builds; keep Buildx + QEMU where a single job builds multi-arch.
  • Remove manual Docker cache settings; builds use Blacksmith cache automatically.
  • Harden .github/actions/setup-bun: install Node 24 when the runner’s Node < 22; expose node-version and min-node-major inputs.

• Bug Fixes

  • Fix invalid workflow YAML from the migration and address unsupported multi-arch paths.

^{Written for commit edb8e9e. Summary will update on new commits.}