os.freemem() on macOS returns only truly free pages (~0.1 GB on a 24 GB
machine), ignoring inactive/purgeable/speculative pages the kernel
reclaims on demand. This caused a false "Low memory" warning on every
macOS machine.
Add getAvailableMemoryMb() that uses vm_stat on macOS and MemAvailable
from /proc/meminfo on Linux, falling back to os.freemem() elsewhere.
Also trim FFmpeg/FFprobe version strings to just "toolname X.Y.Z"
instead of the full copyright line.
