Problem

ScrollableContainerNode.Render() passes the raw outer context directly to ScrolledRenderContext, causing content to always render at absolute terminal row 0 regardless of the node's actual position in the layout tree. This overwrites any content above it (headers, labels, etc.).

Root cause: Every other node calls context.CreateSubContext(bounds) first, which offsets the write origin to bounds.{X,Y}. ScrollableContainerNode was the outlier that skipped this step.

Fix

Call context.CreateSubContext(bounds) before constructing ScrolledRenderContext, consistent with the pattern used by every other node:

-var scrolledContext = new ScrolledRenderContext(context, 0, -_scrollOffset, contentWidth, _contentHeight);
+var boundsContext = context.CreateSubContext(bounds);
+var scrolledContext = new ScrolledRenderContext(boundsContext, 0, -_scrollOffset, contentWidth, _contentHeight);

Also updated the scrollbar drawing to use boundsContext instead of the raw context.

Testing

• ✅ All 1420 existing tests pass (was 1414 + 6 new)
• ✅ 6 new regression tests in ScrollableContainerNodeBoundsTests.cs:
  • Render_AtOffset_PutsContentAtCorrectRow — core fix test
  • Render_InVerticalLayout_PreservesHeaderAboveScrollable — full layout tree
  • Render_ScrollShiftsContentWithinBounds — scrolling still works
  • Render_WrappedInPanelNode_WorkaroundStillWorks — the workaround still works
  • Render_AtRow0_NoHeaderAbove_RendersContent — edge case: no content above
  • Render_WithScrollbar_DrawsScrollbarInBounds — scrollbar position correct

Closes #300