Add cross-platform health check fallbacks

[yiwei888]

Summary

Add cross-platform fallbacks for memory and load health checks when Linux /proc files are unavailable.

Changes

• Keep existing /proc/meminfo and /proc/loadavg behavior where available.
• Add memory fallback using Python standard-library system APIs.
• Add load fallback using os.getloadavg when available.
• Add focused tests that simulate missing /proc files.

Testing

• python -m unittest tools.test_health_check - passed
• python -m py_compile tools\health_check.py tools\test_health_check.py - passed
• python build.py was attempted, but my local Windows environment is missing full project toolchains and npm is not available in PATH, so the full build could not complete.

Checklist

• Relevant modules affected by these changes build locally
• Tests pass locally
• Diagnostic build log is committed in this PR
• Documentation has been updated, if applicable
• Configuration or schema changes are documented, if applicable
• No generated build artifacts are committed, except the required diagnostic build log
• Changes are scoped to the PR purpose and avoid unrelated cleanup
• Security, privacy, and error-handling implications have been considered

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• I would like to request that my diagnostic build log is removed before merging