Add Git overlay, operation tools, and virtualized preview so large folders stay responsive.

Co-authored-by: Cursor <cursoragent@cursor.com>

tests/Explorer.Application.Tests/MarkdownParserTests.cs

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+using Explorer.Application;
+
+namespace Explorer.Application.Tests;
+
+public class MarkdownParserTests
+{
+    [Fact]
+    public void Parses_headings_lists_and_tables()
+    {
+        var doc = MarkdownParser.Parse("""
+            # Title
+
+            Intro paragraph.
+
+            ## Locations
+
+            - One
+            - Two
+
+            | A | B |
+            | --- | --- |
+            | 1 | 2 |
+            """);
+        Assert.Contains(doc.Blocks, b => b is MarkdownHeading h && h.Level == 1 && h.Text == "Title");
+        Assert.Equal("Locations", Assert.Single(doc.Headings).Text);
+        var list = Assert.IsType<MarkdownList>(doc.Blocks.First(b => b is MarkdownList));
+        Assert.Equal(["One", "Two"], list.Items);
+        var table = Assert.IsType<MarkdownTable>(doc.Blocks.First(b => b is MarkdownTable));
+        Assert.Equal(["A", "B"], table.Headers);
+        Assert.Equal(["1", "2"], Assert.Single(table.Rows));
+    }
+
+    [Fact]
+    public void Parses_fenced_code()
+    {
+        var doc = MarkdownParser.Parse("""
+            ```powershell
+            dotnet build
+            ```
+            """);
+        var code = Assert.IsType<MarkdownCode>(Assert.Single(doc.Blocks));
+        Assert.Equal("powershell", code.Language);
+        Assert.Equal("dotnet build", code.Text);
+    }
+
+    [Fact]
+    public void User_guide_in_the_repo_parses()
+    {
+        var dir = new DirectoryInfo(AppContext.BaseDirectory);
+        string? path = null;
+        while (dir is not null)
+        {
+            var candidate = Path.Combine(dir.FullName, "docs", "Documentation.md");
+            if (File.Exists(candidate))
+            {
+                path = candidate;
+                break;
+            }
+
+            dir = dir.Parent;
+        }
+
+        Assert.NotNull(path);
+        var doc = MarkdownParser.Parse(File.ReadAllText(path));
+        Assert.Contains(doc.Headings, h => h.Text == "Locations");
+        Assert.Contains(doc.Headings, h => h.Text == "File Operations Queue");
+        Assert.Contains(doc.Blocks, b => b is MarkdownTable);
+    }
+}