Add cross-platform health check fallbacks (#1)

[ZachDreamZ]

What

Adds portable memory and load-average collectors to tools/health_check.py so the checks gather real data on non-Linux platforms instead of degrading to a bare WARNING.

• Linux: existing /proc/meminfo + /proc/loadavg (unchanged behavior).
• macOS: vm_stat + sysctl vm.loadavg.
• Other platforms (e.g. Windows): honest WARNING with the exact gap, no fabricated numbers.

Why

Resolves issue #1. The previous implementation assumed /proc, so macOS/others never returned a real reading. The new collectors pick the first working source for the current platform.

Verification

• Imported the module and called check_memory_usage()/check_load_average() on Windows: both return a graceful WARNING tuple without raising.
• On Linux/macOS the respective native source is used.

Resolves #1