feat: Add cross-platform health check fallbacks (closes #1)

[royliz3090-jpg]

Cross-Platform Health Check Fallbacks

This PR adds cross-platform fallbacks for memory and load checks in the health check tool.

Changes:

• check_memory_usage(): Added fallbacks via psutil and os.sysconf when /proc/meminfo is unavailable
• check_load_average(): Added fallback via os.getloadavg() when /proc/loadavg is unavailable
• Added comprehensive tests simulating missing /proc files
• Linux behavior remains unchanged

Acceptance Criteria:

• check_memory_usage() keeps Linux path, adds non-Linux fallback
• check_load_average() keeps Linux path, adds os.getloadavg() fallback
• Tests simulate missing /proc files
• JSON/text output backward compatible
• PR explains fallback behavior

Test Results:

test_linux_proc_meminfo ... ok
test_non_linux_fallback_os_sysconf ... ok
test_no_proc_no_psutil ... ok
test_linux_proc_loadavg ... ok
test_os_getloadavg_fallback ... ok
test_no_proc_no_getloadavg ... ok

Ran 6 tests in 0.001s
OK

Closes #1

---

Bounty: $50
Lightning Address: fullphone56@walletofsatoshi.com