fix: persist auto-assigned network subnet across system restarts

[Ronitsabhaya75]

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and Context

Closes #835

In ReservedVmnetNetwork.swift, after retrieving the auto-assigned subnet from macOS, save it to DefaultsStore so it's reused on subsequent restarts.

Code Changes:

// After getting subnet from macOS, persist it
if configuration.subnet == nil {
    DefaultsStore.setOptional(key: .defaultSubnet, value: runningSubnet.description)
}

Testing

• Tested locally
• Added/updated tests
• Added/updated docs