Restore and update legacy KDE game sources

[Daniele-Cangi]

Summary

• recover KBackgammon from KDE's official kdegames 3.5.10 source archive
• add KHangMan using its current KDE Education repository
• document KTron's rename into the existing KSnakeDuel lineage instead of adding a duplicate entry
• leave KAsteroids unchanged because the license/provenance gate did not pass

Evidence

• KDE's KHangMan application page lists current 2026 releases and links to education/khangman; its source declares GPL-2.0-or-later, C++17, CMake, and KF6 dependencies.
• The official KDE kdegames 3.5.10 tarball matches KDE's published SHA1 (287aa0d1ae19b8f04d64c4449fed6a7cec58844d). KBackgammon's source headers declare GPL version 2 or later, and its AUTHORS/source identify Jens Hoefkens.
• KDE's current KSnakeDuel page describes a Tron clone. Repository history contains the KTron-to-KSnakeDuel rename, while current sources retain KTron classes/header lineage and a distinct KSnake mode.
• KAsteroids source headers contain copyright and "Part of KDE" notices, while main.cpp selects a GPL about-dialog license and the package carries GPL COPYING. However, the subtree has no README/AUTHORS or per-file grant resolving the older Debian provenance warning that it was based on an unidentified source. No KAsteroids entry is added, and Sources of KAsteroids #485 is not closed.

Validation

• parsed khangman.md, kbackgammon.md, and ksnakeduel.md with the repository's utils.osg.read_entry
• git diff --cached --check

Closes #486
Closes #487
Closes #488