feat(templates/ui): phase 9 — single-parent editor context

Derive-on-compose guarantees at most one slot owner per template, so the
Parent.* context in the Monaco editor resolves directly via
OwnerCompositionId without a picker. Base templates suppress Parent.*
assistance entirely (empty context).

Removed the multi-parent <select> dropdown from the Add Script form and
the now-redundant _selectedParentIndex / OnParentContextChanged plumbing.
ActiveEditorParent collapses to _editorParents.FirstOrDefault().

src/ScadaLink.CentralUI/Components/Pages/Design/TemplateEdit.razor

@@ -103,20 +103,15 @@
         = Array.Empty<ScadaLink.CentralUI.ScriptAnalysis.CompositionContext>();
 
     /// <summary>
-    /// One entry per template that composes this one (in the design-time
+    /// Editor's Parent.* context. Empty for base templates (no owner exists);
-    /// graph). Populated by <see cref="LoadEditorParentsAsync"/>; the user
+    /// exactly one entry for derived templates — the slot-owner resolved from
-    /// picks one via <see cref="_selectedParentIndex"/> when multiple exist.
+    /// the template's OwnerCompositionId.
     /// </summary>
     private List<ScadaLink.CentralUI.ScriptAnalysis.CompositionContext> _editorParents
         = new();
 
-    /// <summary>Index into <see cref="_editorParents"/>; -1 when none.</summary>
-    private int _selectedParentIndex = -1;
-
     private ScadaLink.CentralUI.ScriptAnalysis.CompositionContext? ActiveEditorParent =>
-        _selectedParentIndex >= 0 && _selectedParentIndex < _editorParents.Count
+        _editorParents.FirstOrDefault();
-            ? _editorParents[_selectedParentIndex]
-            : null;
 
     private bool _showCompForm;
     private int _compComposedTemplateId;
@@ -190,7 +185,6 @@
             // editor.
             _editorChildren = await BuildChildContextsAsync(_compositions);
             _editorParents = await BuildParentContextsAsync(Id);
-            _selectedParentIndex = _editorParents.Count > 0 ? 0 : -1;
 
             _validationResult = null;
         }
@@ -865,21 +859,6 @@
                         </div>
                         <div class="col-12">
                             <label class="form-label">Code</label>
-                            @if (_editorParents.Count > 1)
-                            {
-                                <div class="small text-muted mb-2">
-                                    Parent context for editor:
-                                    <select class="form-select form-select-sm d-inline-block w-auto ms-1"
-                                            value="@_selectedParentIndex"
-                                            @onchange="OnParentContextChanged">
-                                        @foreach (var (parent, idx) in _editorParents.Select((p, i) => (p, i)))
-                                        {
-                                            <option value="@idx">@parent.Name</option>
-                                        }
-                                    </select>
-                                    <span class="ms-2">(this template is composed by @_editorParents.Count parents; pick one for <code>Parent.*</code> assistance)</span>
-                                </div>
-                            }
                             <MonacoEditor @ref="_scriptEditor" Value="@_scriptCode" ValueChanged="@(v => _scriptCode = v)"
                                           Language="csharp" Height="320px"
                                           DeclaredParameters="@ScriptParameterNames.Parse(_scriptParameters)"
@@ -1311,10 +1290,21 @@
 
     private async Task<List<ScadaLink.CentralUI.ScriptAnalysis.CompositionContext>> BuildParentContextsAsync(int templateId)
     {
-        var parents = await TemplateEngineRepository.GetTemplatesComposingAsync(templateId);
+        // Post derive-on-compose: only derived templates have a parent context,
-        return parents
+        // and exactly one — the template that owns their composition slot.
-            .Select(p => BuildCompositionContext(p.Name, p))
+        // Base templates suppress Parent.* assistance.
-            .ToList();
+        if (_selectedTemplate?.IsDerived != true || _ownerTemplate == null)
+            return new List<ScadaLink.CentralUI.ScriptAnalysis.CompositionContext>();
+
+        // Resolve the owner with eager-loaded members so the context has shapes.
+        var owner = await TemplateEngineRepository.GetTemplateByIdAsync(_ownerTemplate.Id);
+        if (owner == null)
+            return new List<ScadaLink.CentralUI.ScriptAnalysis.CompositionContext>();
+
+        return new List<ScadaLink.CentralUI.ScriptAnalysis.CompositionContext>
+        {
+            BuildCompositionContext(owner.Name, owner)
+        };
     }
 
     private static ScadaLink.CentralUI.ScriptAnalysis.CompositionContext BuildCompositionContext(
@@ -1331,12 +1321,6 @@
         return new ScadaLink.CentralUI.ScriptAnalysis.CompositionContext(label, attrs, scripts);
     }
 
-    private void OnParentContextChanged(ChangeEventArgs e)
-    {
-        if (int.TryParse(e.Value?.ToString(), out var idx) && idx >= 0 && idx < _editorParents.Count)
-            _selectedParentIndex = idx;
-    }
-
     private static string MapDataType(ScadaLink.Commons.Types.Enums.DataType dt) => dt switch
     {
         ScadaLink.Commons.Types.Enums.DataType.Boolean => "Boolean",