fix: return string Content from FmScriptCompletionData

The cached TextBlock introduced in f37e5a5 violates Avalonia's
single-parent rule when AvaloniaEdit's CompletionList reuses the
item across multiple ContentPresenter instances — produced an
InvalidOperationException ('already has a visual parent
ContentPresenter') the moment the completion window opened.

Return the bare Text string instead. ContentPresenter wraps it in
its own TextBlock automatically and owns that wrapper exclusively,
so reuse across virtualised rows is safe. Drops one allocation per
Content access compared to the original 'new TextBlock { Text = .. }'
factory pattern.

Author: fuzzzerd

src/SharpFM/Scripting/Editor/FmScriptCompletionData.cs

@@ -1,7 +1,6 @@
 using System;
 using System.Diagnostics.CodeAnalysis;
 using System.Text.RegularExpressions;
-using Avalonia.Controls;
 using Avalonia.Media;
 using AvaloniaEdit.CodeCompletion;
 using AvaloniaEdit.Document;
@@ -17,7 +16,6 @@ public class FmScriptCompletionData : ICompletionData
         new(@"\$\{(\d+):([^}]*)\}|\$0", RegexOptions.Compiled);
 
     private readonly string? _snippet;
-    private readonly TextBlock _content;
 
     public FmScriptCompletionData(string text, string? description = null,
         double priority = 0, string? snippet = null)
@@ -26,18 +24,15 @@ public FmScriptCompletionData(string text, string? description = null,
         Description = description ?? text;
         Priority = priority;
         _snippet = snippet;
-
-        // Cache the Content control instead of allocating a fresh TextBlock
-        // on every property access. AvaloniaEdit's CompletionList may
-        // re-read Content during virtualization / filter narrowing, and
-        // each completion item is owned by a single ContentPresenter so
-        // sharing a per-item TextBlock is safe.
-        _content = new TextBlock { Text = text };
     }
 
     public IImage? Image => null;
     public string Text { get; }
-    public object Content => _content;
+    // Return the bare string. ContentPresenter wraps it in a TextBlock
+    // automatically and that wrapper is owned by exactly one parent, so
+    // unlike a cached Control instance there's no "already has a visual
+    // parent" failure when the completion window reopens or virtualises.
+    public object Content => Text;
     public object Description { get; }
     public double Priority { get; }