Cache CI build outputs

[wasphin]

What problem does this PR solve?

Issue Number: resolve

Problem Summary:
CI recompiles the same sources, downloads the same Bazel dependencies, and
runs four CMake configurations serially on every run, making build validation
unnecessarily slow.

What is changed and the side effects?

Changed:

• Add a shared composite action that configures ccache for Make/CMake builds.
• Cache Bazel repository downloads separately from Bazel build outputs.
• Reuse caches in Linux and macOS build jobs.
• Report ccache statistics so cache behavior can be verified from CI logs.
• Run the four Linux CMake compiler/configuration combinations as independent
  matrix jobs.
• Give each ccache job/configuration an unambiguous namespace so restore
  prefixes cannot select another job's cache.

Measured results:

• Warm Make and CMake compilation jobs complete in about 2-6 minutes, compared
  with baseline averages of 16-21 minutes.
• Cross-commit ccache restores reached 99.2%-99.6% lookup hit rates for the
  correctly matched compiler jobs.
• Warm Bazel jobs complete in about 1-2 minutes, compared with baseline
  averages of 11-38 minutes.
• The Linux and macOS workflows completed successfully after cache
  instrumentation was added.

Side effects:

• Performance effects: Subsequent CI runs can reuse compiler outputs and Bazel
  downloads/build outputs. Ccache is bounded to 2 GiB. Bazel repository caches
  are shared per platform, while build caches remain separated by job. Bazel
  may reuse test results when all relevant inputs are unchanged, following its
  normal disk-cache semantics.

• Breaking backward compatibility: None.

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Check List:

• Please make sure your changes are compilable.
• When providing us with a new feature, it is best to add related tests.
• Please follow Contributor Covenant Code of Conduct.