Add experimental VM networking opt-in for Hyper-V builds

The Hyper-V switch selection in the UI previously did not connect the build VM to the network during provisioning. Since internet-connected Sysprep and capture flows are still experimental, this introduces an explicit "Enable VM Networking" checkbox to the Hyper-V Settings page that defaults to off.

- Adds the `-EnableVMNetworking` parameter to BuildFFUVM.ps1 to conditionally attach the Hyper-V network adapter during VM creation.
- Persists the setting through FFU config files and blocks UI execution if enabled without a valid switch.
- Refactors WPF event handlers in FFUUI.Core to fix dropdown scoping errors.
- Updates documentation and the sample configuration file to reflect the new behavior.

FFUDevelopment/BuildFFUVM_UI.ps1

@@ -425,6 +425,13 @@ $script:uiState.Controls.btnRun.Add_Click({
                 return
             }
 
+            if ($config.EnableVMNetworking -and $config.InstallApps -and [string]::IsNullOrWhiteSpace([string]$config.VMSwitchName)) {
+                [System.Windows.MessageBox]::Show("Select or enter a VM Switch Name before enabling VM networking.", "VM Switch Required", "OK", "Warning") | Out-Null
+                $btnRun.IsEnabled = $true
+                $script:uiState.Controls.txtStatus.Text = "Build canceled: VM switch required for experimental networking."
+                return
+            }
+
             $configFilePath = Join-Path $config.FFUDevelopmentPath "\config\FFUConfig.json"
             # Sort top-level keys alphabetically for consistent output
             $sortedConfig = [ordered]@{}