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/MarqueeRangeTests.cs

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+using Explorer.Application;
+
+namespace Explorer.Application.Tests;
+
+public class MarqueeRangeTests
+{
+    [Fact]
+    public void Stack_selects_rows_the_band_touches()
+    {
+        Assert.Equal([0, 1, 2], MarqueeRange.Stack(10, 70, 10, 24));
+        Assert.Equal([1], MarqueeRange.Stack(24, 30, 10, 24));
+        Assert.Equal([0], MarqueeRange.Stack(10, 10.2, 10, 24));
+    }
+
+    [Fact]
+    public void Wrap_selects_tiles_the_band_covers()
+    {
+        // 3 columns, 40x40 cells. Band over the first two tiles of row 0 and the first of row 1.
+        var hits = MarqueeRange.Wrap(5, 5, 55, 45, count: 8, columns: 3, itemWidth: 40, itemHeight: 40);
+        Assert.Equal([0, 1, 3, 4], hits);
+    }
+
+    [Fact]
+    public void Wrap_ignores_cells_past_the_item_count()
+    {
+        var hits = MarqueeRange.Wrap(0, 0, 120, 40, count: 2, columns: 3, itemWidth: 40, itemHeight: 40);
+        Assert.Equal([0, 1], hits);
+    }
+}