Let practice.json add fields entity.json doesn't have, not just replace them

Merge-Config assigned override values with '=', which throws
SetValueInvocationException when the property doesn't exist on the base
object - so a practice.json could only override fields already present in
entity.json. AJMATS's practice.json supplies insurance_path/pdf_path that
CAMBS's entity.json lacks entirely, which crashed the merge. Use Add-Member
-Force (handles both add and replace), copy whole sections missing from the
base, skip _comment keys, and replace scalar sections directly.

engine.ps1

@@ -97,10 +97,16 @@ function Merge-Config($base, $override) {
     if (-not $override) { return $base }
     $merged = $base | ConvertTo-Json -Depth 10 | ConvertFrom-Json
     foreach ($section in $override.PSObject.Properties) {
-        if ($null -ne $merged.PSObject.Properties[$section.Name]) {
+        if ($section.Name -like "_*") { continue }
+        $baseSection = $merged.PSObject.Properties[$section.Name]
+        if ($null -eq $baseSection) {
+            $merged | Add-Member -NotePropertyName $section.Name -NotePropertyValue $section.Value
+        } elseif ($section.Value -is [System.Management.Automation.PSCustomObject]) {
             foreach ($prop in $section.Value.PSObject.Properties) {
-                $merged.$($section.Name).$($prop.Name) = $prop.Value
+                $merged.$($section.Name) | Add-Member -NotePropertyName $prop.Name -NotePropertyValue $prop.Value -Force
             }
+        } else {
+            $merged.$($section.Name) = $section.Value
         }
     }
     return $merged