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

tests/ScadaLink.TemplateEngine.Tests/TemplateNamingTests.cs

@@ -0,0 +1,87 @@
+using ScadaLink.Commons.Entities.Templates;
+
+namespace ScadaLink.TemplateEngine.Tests;
+
+/// <summary>
+/// Coverage for <see cref="TemplateNaming.QualifiedName"/> — the computed
+/// hierarchical name of a composition-derived template. Derived templates store
+/// only their contained name (the composition slot's <c>InstanceName</c>); the
+/// dotted path is resolved on read by walking the <c>OwnerCompositionId</c> chain.
+/// </summary>
+public class TemplateNamingTests
+{
+    private static (Dictionary<int, Template> byId, Dictionary<int, TemplateComposition> compById)
+        BuildGraph(params Template[] templates)
+    {
+        var byId = templates.ToDictionary(t => t.Id);
+        var compById = templates
+            .SelectMany(t => t.Compositions)
+            .ToDictionary(c => c.Id);
+        return (byId, compById);
+    }
+
+    [Fact]
+    public void QualifiedName_BaseTemplate_IsJustItsName()
+    {
+        var motorController = new Template("Motor Controller") { Id = 4 };
+        var (byId, compById) = BuildGraph(motorController);
+
+        Assert.Equal("Motor Controller", TemplateNaming.QualifiedName(motorController, byId, compById));
+    }
+
+    [Fact]
+    public void QualifiedName_OneLevelDerived_PrefixesTheOwner()
+    {
+        // Motor Controller composes the Pump template into a slot named "Pump".
+        var motorController = new Template("Motor Controller") { Id = 4 };
+        motorController.Compositions.Add(
+            new TemplateComposition("Pump") { Id = 1014, TemplateId = 4, ComposedTemplateId = 2018 });
+        var derivedPump = new Template("Pump")
+        {
+            Id = 2018, IsDerived = true, OwnerCompositionId = 1014
+        };
+        var (byId, compById) = BuildGraph(motorController, derivedPump);
+
+        Assert.Equal("Motor Controller.Pump", TemplateNaming.QualifiedName(derivedPump, byId, compById));
+    }
+
+    [Fact]
+    public void QualifiedName_NestedDerived_WalksTheWholeChain()
+    {
+        // Motor Controller -> Pump slot -> TempSensor slot.
+        var motorController = new Template("Motor Controller") { Id = 4 };
+        motorController.Compositions.Add(
+            new TemplateComposition("Pump") { Id = 1014, TemplateId = 4, ComposedTemplateId = 2018 });
+
+        var derivedPump = new Template("Pump")
+        {
+            Id = 2018, IsDerived = true, OwnerCompositionId = 1014
+        };
+        derivedPump.Compositions.Add(
+            new TemplateComposition("TempSensor") { Id = 1015, TemplateId = 2018, ComposedTemplateId = 2019 });
+
+        var derivedTempSensor = new Template("TempSensor")
+        {
+            Id = 2019, IsDerived = true, OwnerCompositionId = 1015
+        };
+        var (byId, compById) = BuildGraph(motorController, derivedPump, derivedTempSensor);
+
+        Assert.Equal(
+            "Motor Controller.Pump.TempSensor",
+            TemplateNaming.QualifiedName(derivedTempSensor, byId, compById));
+    }
+
+    [Fact]
+    public void QualifiedName_DerivedWithMissingOwnerLink_FallsBackToStoredName()
+    {
+        // Defensive: a derived template whose owner composition is not in the
+        // lookup must not throw — it falls back to the stored contained name.
+        var orphan = new Template("TempSensor")
+        {
+            Id = 2019, IsDerived = true, OwnerCompositionId = 9999
+        };
+        var (byId, compById) = BuildGraph(orphan);
+
+        Assert.Equal("TempSensor", TemplateNaming.QualifiedName(orphan, byId, compById));
+    }
+}