Add cross-platform health check fallbacks (#1)

[xudexing19920829]

Add cross-platform health check fallbacks

Fixes: #1

Changes

check_memory_usage() now has a cross-platform fallback:

• Primary path (Linux): Reads /proc/meminfo as before — no change in behavior
• macOS fallback (_check_memory_fallback): Uses sysctl -n hw.memsize for total memory and vm_stat for page-level available memory. Calculates used percentage and returns same (status, detail, pct) tuple
• Linux fallback (no /proc): Uses free -b command
• Returns "WARNING" with descriptive message if no method works

check_load_average() now has a cross-platform fallback:

• Primary path (Linux): Reads /proc/loadavg as before — no change
• Fallback (_check_load_average_fallback): Uses os.getloadavg() which works on macOS, Linux, and BSD. Returns same tuple format with load percentages
• Returns "WARNING" on platforms without getloadavg (Windows)

Additional improvements:

• Added platform.system() to health check output for visibility
• Platform info included in JSON output
• Print report now shows platform

Acceptance criteria

• ✅ check_memory_usage() keeps existing /proc/meminfo path on Linux
• ✅ macOS fallback returns meaningful memory status via sysctl/vm_stat
• ✅ check_load_average() keeps existing /proc/loadavg path where available
• ✅ Falls back to os.getloadavg() on non-Linux Unix systems
• ✅ JSON and text output remain backward compatible
• ✅ All fallbacks return same (status, detail, value) tuple format