chore: update BCR maintainer and CI runners

[longlho]

Summary:

• Add pplx-oss as a BCR maintainer with ossmaintainers@perplexity.ai and its GitHub user id.
• Run compile-heavy GitHub Actions jobs on ubuntu-24.04-16core.
• Move macOS coverage to a non-PR //py:py_test smoke while darwin-target cross-compile analysis stays on Linux.
• Pin workflow actions to full commit SHAs so the org Actions policy allows CI to run.
• Update the README CI summary to match the workflow.

Validation:

• git diff --check
• ruby -rjson -e "JSON.parse(File.read(".bcr/metadata.template.json"))"
• ruby -ryaml -e "YAML.load_file(".github/workflows/ci.yaml")"
• rg -n "ubuntu-latest|ubuntu-24.04|macos-smoke|macos-latest|uses: actions/checkout@|uses: bazel-contrib/setup-bazel@|bazel test //...|//py:py_test" .github/workflows README.md