feat(template-engine): contained names for composition-derived templates

A composition-derived template now stores its contained name — the
composition slot's InstanceName (e.g. "Pump"), unique only within its
owner — instead of the dotted global path ("Motor Controller.Pump").
The qualified hierarchical name is computed on read.

- TemplateNaming.QualifiedName: walks the OwnerCompositionId chain to
  build the dotted path; null-safe, cycle-guarded.
- TemplateConfiguration: the unique index on Template.Name becomes
  filtered (WHERE IsDerived = 0) — base templates stay globally unique;
  derived templates' uniqueness is the existing (TemplateId,
  InstanceName) index on TemplateComposition.
- Migration ContainedDerivedTemplateNames: rewrites derived rows to the
  contained name; Down rebuilds the dotted names via a recursive CTE
  before restoring the global index.
- TemplateService: composition create/rename store the contained name;
  the dotted-name collision pre-checks and cascade-rename are removed
  (a slot rename no longer touches nested derived templates).
- TemplateEdit: title shows the contained name; the qualified path is a
  breadcrumb subtitle; "composed inside" uses the owner's qualified name.

TDD: 4 TemplateNaming tests + updated composition tests. TemplateEngine
293, ConfigurationDatabase 114, CentralUI 316 green. Migration applied to
the dev cluster and verified in the browser (Motor Controller.Pump now
titled "Pump"; nested Motor Controller.Pump.TempSensor resolves).

Design: docs/plans/2026-05-18-contained-template-names-design.md

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

@@ -236,7 +236,7 @@
                     <a href="/design/templates/@_baseTemplate.Id"><code>@_baseTemplate.Name</code></a>
                     @if (_ownerTemplate != null && _ownerComposition != null)
                     {
-                        <span class="ms-1">— composed inside <a href="/design/templates/@_ownerTemplate.Id"><code>@_ownerTemplate.Name</code></a> as <code>@_ownerComposition.InstanceName</code>.</span>
+                        <span class="ms-1">— composed inside <a href="/design/templates/@_ownerTemplate.Id"><code>@QualifiedTemplateName(_ownerTemplate)</code></a> as <code>@_ownerComposition.InstanceName</code>.</span>
                     }
                 </div>
             </div>
@@ -248,6 +248,12 @@
                 {
                     <span class="text-muted ms-2">inherits @(_templates.FirstOrDefault(t => t.Id == _selectedTemplate.ParentTemplateId)?.Name)</span>
                 }
+                @if (_selectedTemplate.IsDerived)
+                {
+                    @* Derived templates store a contained name; show the full
+                       qualified path as a breadcrumb subtitle. *@
+                    <div class="text-muted small font-monospace">@QualifiedTemplateName(_selectedTemplate)</div>
+                }
             </div>
             <div>
                 <button class="btn btn-outline-info btn-sm me-1" @onclick="RunValidation" disabled="@_validating">
@@ -1701,6 +1707,18 @@
         else { _toast.ShowError(result.Error); }
     }
 
+    /// <summary>
+    /// Computes a template's qualified (hierarchical) name from the loaded
+    /// template set — the stored name for a base template, the dotted
+    /// owner-chain path for a composition-derived one.
+    /// </summary>
+    private string QualifiedTemplateName(Template template)
+    {
+        var byId = _templates.ToDictionary(t => t.Id);
+        var compById = _templates.SelectMany(t => t.Compositions).ToDictionary(c => c.Id);
+        return TemplateNaming.QualifiedName(template, byId, compById);
+    }
+
     // ---- Editor metadata builders ----
 
     private async Task<IReadOnlyList<ScadaLink.CentralUI.ScriptAnalysis.CompositionContext>> BuildChildContextsAsync(