refactor(scripting): remove catalog infrastructure now that every step is a typed POCO

All 205 FileMaker script steps are IStepFactory POCOs discovered via
reflection in StepRegistry. The catalog-driven layer that preceded them
has nothing left to serve, so it comes out entirely.

Deleted:
- step-catalog-en.json embedded resource (−8,306 lines).
- StepCatalog.cs (StepDefinition, StepParam, enum converters).
- StepCatalogLoader.cs (JSON loader + ByName/ById indices).
- IStepCatalog.cs contract.
- CatalogXmlBuilder.cs (XElement synthesis from param maps).
- CatalogDisplayRenderer.cs (generic display-line renderer).
- CatalogValidator.cs (generic per-param validator).
- CatalogParamExtractor.cs (helper for reading values out of XML).

Kept as forward-compat surface:
- RawStep wraps elements whose name isn't in the registry (e.g. a step
  introduced by a future FileMaker release). It's sealed in the display
  editor and surfaced with a yellow-underline warning pointing users to
  the XML editor; round-trip remains byte-intact.
- StepBlockPair + BlockPairRole recreated in BlockPair.cs — consumed by
  StepMetadata.BlockPair for indent / pair-match logic.

Migrated to StepRegistry:
- ScriptValidator.Validate now looks up StepMetadata + ParamMetadata.
- FmScript.ToDisplayLines reads BlockPair through StepRegistry.MetadataFor.
- FmScript.Apply(Add|Update) route new steps through POCO factories.
- ScriptTextParser drops its catalog fallback; unknown names wrap in RawStep.

Other moves:
- ScriptStep.Definition property removed. Every POCO ctor now calls
  base(enabled) only. 198 POCOs mechanically updated.
- CommentStep upgraded to IStepFactory.
- Unknown-step diagnostic severity flipped Error → Warning with an
  'edit via the XML editor' message.

Author: fuzzzerd

src/SharpFM.Model/Scripting/BlockPair.cs

@@ -0,0 +1,27 @@
+namespace SharpFM.Model.Scripting;
+
+/// <summary>
+/// Role of a step in a block-pair construct (if/else/end-if, loop/end-loop).
+/// </summary>
+public enum BlockPairRole
+{
+    Open,
+    Middle,
+    Close,
+}
+
+/// <summary>
+/// Block-pair metadata carried by step POCOs that open, continue, or
+/// close a block. Drives indentation in <see cref="FmScript.ToDisplayLines"/>
+/// and matching-partner validation in <see cref="ScriptValidator"/>.
+/// </summary>
+public sealed record StepBlockPair
+{
+    public required BlockPairRole Role { get; init; }
+
+    /// <summary>
+    /// Canonical names of the other steps that participate in this
+    /// block. E.g. an If's partners are ["Else", "Else If", "End If"].
+    /// </summary>
+    public required string[] Partners { get; init; }
+}