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/FolderDisplayRefreshTests.cs

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+using Explorer.Application;
+using Explorer.Domain;
+
+namespace Explorer.Application.Tests;
+
+public class FolderDisplayRefreshTests
+{
+    [Fact]
+    public void Pick_prefers_visible_then_largest()
+    {
+        var items = new[]
+        {
+            Dir(@"C:\a", "a", 50_000_000),
+            Dir(@"C:\b", "b", 90_000_000),
+            Dir(@"C:\c", "c", 5_000),
+            Dir(@"C:\d", "d", 20_000_000)
+        };
+        var picked = FolderDisplayRefresh.Pick(items, [@"C:\d", @"C:\a"]);
+        Assert.Equal(3, picked.Count);
+        Assert.Equal(@"C:\a", picked[0].FullPath);
+        Assert.Equal(@"C:\d", picked[1].FullPath);
+        Assert.Equal(@"C:\b", picked[2].FullPath);
+    }
+
+    [Fact]
+    public void Pick_caps_and_skips_tiny_or_cloud()
+    {
+        var items = Enumerable.Range(0, 12)
+            .Select(i => Dir($@"C:\f{i}", $"f{i}", (13 - i) * 2_000_000L))
+            .Append(Dir(@"C:\tiny", "tiny", 100))
+            .Append(Dir(@"C:\cloud", "cloud", 80_000_000, hydrate: true))
+            .ToList();
+        var picked = FolderDisplayRefresh.Pick(items);
+        Assert.Equal(FolderDisplayRefresh.MaxProbes, picked.Count);
+        Assert.Equal(@"C:\f0", picked[0].FullPath);
+        Assert.DoesNotContain(picked, item => item.Name is "tiny" or "cloud");
+    }
+
+    [Fact]
+    public void NeedsVerify_compares_child_counts()
+    {
+        Assert.True(FolderDisplayRefresh.NeedsVerify(10, 2000));
+        Assert.False(FolderDisplayRefresh.NeedsVerify(10, 10));
+        Assert.False(FolderDisplayRefresh.NeedsVerify(-1, 2000));
+    }
+
+    [Fact]
+    public void HasIndexCounts_requires_index_hydration()
+    {
+        var live = Dir(@"C:\a", "a", 50_000_000, hydration: ItemHydrationFlags.Shell | ItemHydrationFlags.Metadata);
+        Assert.False(FolderDisplayRefresh.HasIndexCounts(live));
+        Assert.True(FolderDisplayRefresh.HasIndexCounts(live.Overlay(hydration: ItemHydrationFlags.Index)));
+    }
+
+    private static FileSystemItem Dir(string path, string name, long size, bool hydrate = false, ItemHydrationFlags? hydration = null)
+        => new()
+        {
+            FullPath = path,
+            Name = name,
+            IsDirectory = true,
+            SizeBytes = size,
+            IndexedChildCount = 4,
+            Hydration = hydration ?? ItemHydrationFlags.All,
+            Cloud = hydrate
+                ? new CloudPresence("x", CloudAvailability.OnlineOnly, size, null, true)
+                : null
+        };
+}