Fix unsigned underflow in available physical page calculation

[cmssmc00]

Partially addresses #371.

Summary:

• Avoid unsigned underflow when subtracting GPU utilization headroom from available physical memory.
• Return 0 usable physical memory when available memory is below the configured headroom.
• Prevent the prealloc path from seeing an artificially huge available-page count under memory pressure.

Tests:

• git diff --check