Show folder names immediately and refresh stale index sizes without walking the whole drive.

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

tests/Explorer.Application.Tests/FolderStatusTextTests.cs

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+using Explorer.Application;
+using Explorer.Domain;
+
+namespace Explorer.Application.Tests;
+
+public class FolderStatusTextTests
+{
+    [Fact]
+    public void Empty_folder_is_zero_items()
+    {
+        Assert.Equal("0 items", FolderStatusText.Format([], []));
+    }
+
+    [Fact]
+    public void Folder_totals_use_known_sizes()
+    {
+        var items = new[]
+        {
+            File("a.txt", 100),
+            File("b.txt", 924),
+            Folder("pics", size: 0, SizeKnowledge.Unknown)
+        };
+
+        Assert.Equal("3 items  ·  1 KB", FolderStatusText.Format(items, []));
+    }
+
+    [Fact]
+    public void Selection_replaces_folder_totals()
+    {
+        var items = new[] { File("a.txt", 100), File("b.txt", 200), File("c.txt", 300) };
+
+        Assert.Equal("1 item selected  ·  200 B", FolderStatusText.Format(items, [items[1]]));
+        Assert.Equal("2 items selected  ·  300 B", FolderStatusText.Format(items, [items[0], items[1]]));
+    }
+
+    [Fact]
+    public void Unknown_folder_sizes_are_omitted()
+    {
+        var folders = new[]
+        {
+            Folder("a", 0, SizeKnowledge.Unknown),
+            Folder("b", 0, SizeKnowledge.Unknown)
+        };
+
+        Assert.Equal("2 items", FolderStatusText.Format(folders, []));
+    }
+
+    [Fact]
+    public void Indexed_folder_sizes_are_included()
+    {
+        var items = new[]
+        {
+            Folder("docs", 2_048, SizeKnowledge.Calculated),
+            File("readme.txt", 1_024)
+        };
+
+        Assert.Equal("2 items  ·  3 KB", FolderStatusText.Format(items, []));
+    }
+
+    [Fact]
+    public void Zero_byte_file_selection_still_shows_size()
+    {
+        var file = File("empty.dat", 0);
+        Assert.Equal("1 item selected  ·  0 B", FolderStatusText.Format([file], [file]));
+    }
+
+    private static FileSystemItem File(string name, long size)
+        => new()
+        {
+            FullPath = @"C:\" + name,
+            Name = name,
+            SizeBytes = size,
+            SizeKnowledge = SizeKnowledge.Calculated
+        };
+
+    private static FileSystemItem Folder(string name, long size, SizeKnowledge knowledge)
+        => new()
+        {
+            FullPath = @"C:\" + name,
+            Name = name,
+            IsDirectory = true,
+            SizeBytes = size,
+            SizeKnowledge = knowledge
+        };
+}