[MAX] Fix macOS type checking for Linux-only OS calls - #7436

Add Linux platform guards to the checkpoint page-cache eviction and precompile worker CPU affinity helpers so macOS mypy checks do not resolve os.posix_fadvise, os.POSIX_FADV_DONTNEED, or os.sched_setaffinity. Linux behavior is unchanged.

Validation: both affected mypy checks and Ruff lint and formatting checks pass on macOS.

Assisted-by: AI

Type of change

• [ X] Bug fix (non-breaking change that fixes an issue)
• Performance improvement (includes benchmark results below)
• Documentation update
• New feature or public API (requires prior proposal or issue approval)
• Refactor / internal cleanup (no user-visible change)
• [ X] Build, CI, or tooling change

Motivation

• macOS builds fail during mypy checks because two helpers reference Linux-only APIs: os.posix_fadvise, os.POSIX_FADV_DONTNEED, and os.sched_setaffinity.
• These helpers need explicit platform guards so type checking succeeds on macOS.

What changed

• Added an early return outside Linux in the checkpoint page-cache eviction helper.
• Added the same guard to the precompilation worker’s CPU-affinity helper.
• Updated the CPU-affinity docstring to state its Linux scope.
• Linux behavior and function signatures remain unchanged.

Testing

• The affected macOS mypy checks pass after reproducing the failures before the fixes.
• Ruff lint and formatting checks pass for both modified files.
• The full repository build completed successfully on macOS.
• No new regression tests were added; validation used the existing type-checking and build targets.

Checklist

• The linked issue above has been reviewed by a maintainer and is
  agreed-upon, or this is a trivial fix that does not need prior
  approval
• [ X] PR is small and focused — I've split larger changes into a sequence of
  smaller PRs where possible (see
  pull request sizes)
• I ran ./bazelw run format to format my changes
• I added or updated tests to cover my changes
• [ X] If AI tools assisted with this contribution, I have included an
  Assisted-by: trailer in my commit message or this PR description (see
  AI Tool Use Policy)