Fix default dockable split

[wieslawsoltes]

PR summary: issue 936 default dockable split fix

Summary

Fixes a layout serialization failure where RootDock.DefaultDockable could point
to a replaced dock after a global split operation. The fix keeps the default
pointer aligned with the new split container and adds a regression test.

Findings

• A global split that replaces a dock in its owner (via SplitToDock) updates
  ActiveDockable but left DefaultDockable pointing at the old dock instance.
• The stale default is serialized, so deserialization reconstructs a layout with
  an orphaned DefaultDockable reference, which can break load or result in an
  invalid root state.
• The issue is visible with a flat root layout (single visible dockable) where
  a split wraps the dock in a new ProportionalDock, leaving the root default
  pointed at the pre-split dock.

Implementation

• FactoryBase.SplitToDock now detects when the owner default is the dock being
  replaced and retargets DefaultDockable to the new split layout.
• The change is limited to the replacement path (non-optimized branch) to avoid
  altering behavior when the owner container is reused.

var wasDefaultDockable = ownerDock.DefaultDockable == dock;
...
ownerDock.ActiveDockable = layout;
if (wasDefaultDockable)
{
    ownerDock.DefaultDockable = layout;
}

• Added a headless test to assert both ActiveDockable and DefaultDockable
  point to the new layout after a split.

Usage guide

• No API changes are required. Update to a build that includes this fix.
• If you rely on RootDock.DefaultDockable to restore the main layout on load,
  it will now follow split replacements automatically.
• To verify in DockMvvmSample, use the issue repro steps: perform a global
  split on the root, save the layout, then reload. The layout should deserialize
  correctly and the root default should match the split container.
• If you implement custom split logic outside FactoryBase.SplitToDock, mirror
  this behavior by updating DefaultDockable when a dock is replaced.

Tests

• dotnet test

Fixes #936