Add cross-platform health check fallbacks

[bingmokaka]

Summary

Adds cross-platform fallbacks for the health check memory and load probes so non-Linux environments can report useful diagnostics when /proc is unavailable.

Changes

• Kept the existing Linux /proc/meminfo and /proc/loadavg paths intact.
• Added a Windows memory fallback using standard-library ctypes and GlobalMemoryStatusEx.
• Added a POSIX-style memory fallback using os.sysconf where available.
• Added a load average fallback using os.getloadavg() where available.
• Added focused unittest coverage that simulates missing /proc files and verifies fallback behavior.

Testing

• python -m unittest tests.test_health_check_fallbacks -v passes: 5 tests.
• python tools/health_check.py --json --service backend preserves JSON output shape; memory reports a meaningful Windows value.
• PYTHONIOENCODING=utf-8 python tools/health_check.py --service backend preserves text output shape; memory reports a meaningful Windows value.
• python build.py was attempted but the Windows environment lacks the full multi-language toolchain and npm, so the all-module build crashes when spawning npm install for frontend.
• python build.py -m backend was also attempted; it reached diagnostic finalization but failed because the bundled encryptly diagnostic pack step returned an error in this Windows environment. No new diagnostic artifact was committed.

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

Closes #1