Add cross-platform health check fallbacks

[daxia778]

Addresses #1

Summary

Adds cross-platform fallback paths for tools/health_check.py so memory and load checks no longer degrade to generic /proc read warnings on non-Linux hosts.

Changes

• Keeps the existing /proc/meminfo and /proc/loadavg behavior as the first path on Linux.
• Adds memory fallback support for macOS (sysctl + vm_stat), Windows (GlobalMemoryStatusEx), and generic POSIX os.sysconf.
• Adds load average fallback via os.getloadavg() when /proc/loadavg is unavailable.
• Adds focused unittest coverage for missing-/proc memory/load fallback behavior and health-check output shape.
• Commits matching diagnostic artifacts for the code commit:
  • diagnostic/build-f1cce566-metadata.json
  • diagnostic/build-f1cce566-part001.logd
  • diagnostic/build-f1cce566-part002.logd

Testing

• python3 -m py_compile tools/health_check.py tests/test_health_check_fallbacks.py — passed.
• python3 -m unittest discover -s tests -v — 4 tests passed.
• Manual macOS check: check_memory_usage() now returns a measured value instead of a /proc warning.
• /opt/homebrew/bin/python3.12 build.py — ran full build and finalized diagnostics. Local result was 6 passed / 4 failed: frailbox fails on an existing C compile error, and nfc-scanner, openapi-haskell, and openapi-tools fail because luac/ghc are not installed locally. The diagnostic metadata and .logd chunks from that run are committed.
• Reassembled and unpacked the diagnostic chunks locally with the recorded password to confirm they are decryptable.
• git diff --check — passed.

Checklist

• Relevant Python validation passes 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

---

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

Payment preference after acceptance: USDC on Base to 0xB54Ec1F0905186553FaB1763409e1CC127b874F9. If the project uses a different approved payout rail, I can follow that flow.