Add public access to child layout data in UIWidget

[pvcraven]

Summary

• Adds get_child_data(child) to UIWidget — returns the live layout kwargs dict (e.g., anchor_x, align_x) for a child widget, or None if not found
• Adds get_child_entry(index) to UIWidget — returns (child, data) tuple by index, supporting negative indices
• Modifying the returned dict affects layout on the next do_layout() call, enabling in-place repositioning without remove/re-add cycles

Test plan

• get_child_data() returns correct dict for added child
• get_child_data() returns None for unknown widget
• Modifying returned dict affects layout positioning after do_layout()
• get_child_entry(0) returns first child with data
• get_child_entry(-1) returns last child (Python index semantics)
• get_child_entry(out_of_range) raises IndexError
• Works with UIAnchorLayout, UIBoxLayout, UIGridLayout
• After remove() + add(), returns new data
• All 72 existing GUI layout tests pass

🤖 Generated with Claude Code