.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 all selected GitHub Actions workflows to Blacksmith runners to speed up CI and improve caching and observability. No job logic changed; only runs-on targets were updated.

• Migration
  • Replaced GitHub-hosted runners with blacksmith-* tags: blacksmith-4vcpu-ubuntu-2404, blacksmith-4vcpu-ubuntu-2404-arm, blacksmith-4vcpu-ubuntu-2204, blacksmith-6vcpu-macos-15, blacksmith-4vcpu-windows-2025.
  • Updated Linux, ARM, macOS, and Windows jobs across workflows (builds, tests, docs, integrations, cross-compiles).
  • Containers, matrices, and steps remain the same.
  • Gains: faster machines, colocated Actions cache, shared Docker layer cache, and better logs/observability.

^{Written for commit 585b4a8. Summary will update on new commits.}