fix: clone tabs instead of reparenting

[MelkorBalrog]

Summary

• rebuild detached tabs by cloning widgets rather than reparenting across toplevel windows
• update regression test to verify tab cloning behaviour
• document the change in HISTORY

Testing

• radon cc -j gui/utils/widget_transfer_manager.py
• pytest tests/detachment/window/test_reparent_across_toplevel.py
• pytest (failed: Segmentation fault)

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https://chatgpt.com/codex/tasks/task_b_68bba11ff6c08327b511e5b78c9ab667

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

[P1] Cloned tabs discard widget runtime state

The new _clone_widget instantiates fresh widgets and only replays configuration options and event bindings. Stateful content (text inserted into a Text/Entry without a textvariable, listbox items, canvas drawings, etc.) is stored inside the widget rather than in configure() data, so these values are dropped when the original tab is destroyed and the clone is added. The previous implementation reparented the existing widget and preserved this state. Detaching a tab that users have already interacted with will now silently lose their data.

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