Fix PaintHarness after rebase: paint from the fragment tree, not deleted CssBox.Paint

PR ArthurHub#262's PaintHarness.PaintBox (merged into master while this branch was in
flight) called box.Paint(g) directly - the old live-tree paint entry point.
This branch's own F3 stage later deleted CssBox.Paint/PaintImp entirely once
FragmentPainter became the sole paint path, so rebasing this branch onto
master (which now carries PaintHarness) fails to compile: git's line-based
merge can't catch an API a fresh commit calls having been removed by an
earlier one in this branch's own history.

Fix: PaintHarness.PaintBox now locates the CssBox's own BoxFragment in
container.FragmentTree (searching every fragmentainer, in case the box was
relocated onto a later page) and paints it via a new FragmentPainter test
entry point, FragmentPainter.PaintFragmentSubtree - a thin wrapper that sets
the painter's band-top before delegating to its existing private PaintFragment,
letting a test get "just this box's draw calls" without duplicating the real
paint walk. This is the same widen-for-test-use precedent already applied to
CssBox.PaintBackground/PaintWord/PaintDecoration.

Full three-project suite passes after the rebase: HtmlRenderer.Test (2367),
HtmlRenderer.IntegrationTest (146), HtmlRenderer.PdfSharp.Test (30) - 0 failed.

Author: jhaygood86

Source/HtmlRenderer/Core/Paint/FragmentPainter.cs

@@ -123,6 +123,19 @@ internal void Paint(RGraphics g, FragmentainerFragment fragmentainer)
             PaintFragment(g, fragmentainer.Root);
         }
 
+        /// <summary>
+        /// Test-support entry point: paints one box fragment (and its descendants) directly, without
+        /// painting the rest of its fragmentainer - mirrors what the old live-tree <c>CssBox.Paint(g)</c>
+        /// did for an arbitrary box, for tests that want the draw-call log of just one subtree.
+        /// <paramref name="bandTop"/> should be the owning fragmentainer's own
+        /// <see cref="FragmentainerFragment.LocalOriginY"/>.
+        /// </summary>
+        internal void PaintFragmentSubtree(RGraphics g, BoxFragment fragment, double bandTop = 0)
+        {
+            _bandTop = bandTop;
+            PaintFragment(g, fragment);
+        }
+
         /// <summary>
         /// Paints one box fragment: display/visibility gate, fixed-position clip suspension, and the
         /// same "is this rect actually in the visible area" cull the old live-tree walk used, before

Source/Test/HtmlRenderer.IntegrationTest/TestSupport/PaintHarness.cs

@@ -1,6 +1,8 @@
 using TheArtOfDev.HtmlRenderer.Adapters.Entities;
 using TheArtOfDev.HtmlRenderer.Core;
 using TheArtOfDev.HtmlRenderer.Core.Dom;
+using TheArtOfDev.HtmlRenderer.Core.Fragments;
+using TheArtOfDev.HtmlRenderer.Core.Paint;
 
 namespace HtmlRenderer.IntegrationTest.TestSupport;
 
@@ -68,8 +70,45 @@ internal static void PaintBox(HtmlContainerInt container, CssBox box, RecordingG
             g.PushClip(new RRect(container.MarginLeft, container.MarginTop, container.PageSize.Width, container.PageSize.Height));
         }
 
-        box.Paint(g);
+        var (fragment, bandTop) = FindFragment(container, box);
+        new FragmentPainter(container).PaintFragmentSubtree(g, fragment, bandTop);
 
         g.PopClip();
     }
+
+    /// <summary>
+    /// Locates <paramref name="box"/>'s own <see cref="BoxFragment"/> in <paramref name="container"/>'s
+    /// fragment tree (built by <see cref="HtmlContainerInt.PerformLayout"/>), searching every fragmentainer
+    /// since a box relocated onto a later page won't be found on the first one. Paint now reads geometry
+    /// from the fragment tree exclusively (<c>CssBox.Paint</c>/<c>PaintImp</c> were deleted once
+    /// <see cref="FragmentPainter"/> became the only paint path), so a harness that wants "the draw calls
+    /// for this one box" has to find its fragment first, the same way <see cref="FragmentPainter.Paint"/>
+    /// itself starts from a fragmentainer's own <c>Root</c> fragment rather than a live <c>CssBox</c>.
+    /// </summary>
+    private static (BoxFragment Fragment, double BandTop) FindFragment(HtmlContainerInt container, CssBox box)
+    {
+        foreach (var fragmentainer in container.FragmentTree.Fragmentainers)
+        {
+            var found = FindFragment(fragmentainer.Root, box);
+            if (found != null)
+                return (found, fragmentainer.LocalOriginY);
+        }
+
+        throw new InvalidOperationException("No fragment found for the given box - is it display:none, or otherwise never laid out?");
+    }
+
+    private static BoxFragment? FindFragment(BoxFragment fragment, CssBox box)
+    {
+        if (ReferenceEquals(fragment.Box, box))
+            return fragment;
+
+        foreach (var child in fragment.Children)
+        {
+            var found = FindFragment(child, box);
+            if (found != null)
+                return found;
+        }
+
+        return fragment.MarkerFragment != null ? FindFragment(fragment.MarkerFragment, box) : null;
+    }
 }