[Code scan] Include the GPLv3 companion text for LGPLv3 licensing

[njzjz]

This issue was found during a Codex global code scan of the repository.

Baseline commit: e3c5b38

Problem

The repository contains the LGPLv3 text in LICENSE, but the LGPLv3 text incorporates and refers to the GNU GPL. A separate GPLv3 companion text was not present in the tracked files.

Code references:

dpgui/LICENSE

Lines 9 to 11 in e3c5b38

	This version of the GNU Lesser General Public License incorporates
	the terms and conditions of version 3 of the GNU General Public
	License, supplemented by the additional permissions listed below.

dpgui/LICENSE

Lines 76 to 77 in e3c5b38

	b) Accompany the object code with a copy of the GNU GPL and this license
	document.

dpgui/LICENSE

Lines 91 to 92 in e3c5b38

	b) Accompany the Combined Work with a copy of the GNU GPL and this license
	document.

Impact

This is license hygiene rather than a runtime failure. Downstream redistributors and source package consumers may expect both GPLv3 and LGPLv3 texts to be present when the project is licensed under LGPLv3.

Suggested fix

Add the GPLv3 text as COPYING and keep the LGPLv3 text as COPYING.LESSER, or use another conventional file layout that includes both license texts in source and release artifacts.