Fix splash screen animation cleanup

[MelkorBalrog]

Summary

• cancel splash screen animation and fade timers before destroying the window to avoid orphaned Tcl callbacks
• add regression coverage that verifies splash screen shutdown removes _animate callbacks
• document the fix in HISTORY.md with a new 0.2.280 entry

Testing

• pytest (fails in this environment with a segmentation fault in tests/api/test_core_api.py)
• pytest tests/detachment/after_callbacks/test_splash_screen_cleanup.py -k splash

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https://chatgpt.com/codex/tasks/task_b_68e66b454bd083278437c9477ed1f1d3