test: validate EndKeep on real BDS with Python 3.15

[ReallocAll]

Summary

• Add a targeted, platform-matrix real BDS integration test for EndKeep and CPython 3.15 on Linux and Windows x64.
• Obtain the exact successful upstream Endstone Build run 38045137884 (0.11.14.dev6, Minecraft 26.52) using the lab's existing REPO_PAT token.
• Repackage the uploaded non-archived Endstone artifact's original Wheel members for installation, avoiding accidental installation of another Python ABI.
• Verify a fresh EndKeep dependency resolution, Amulet-LevelDB version, EndKeep plugin .whl loading (without duplicate editable installation), /backup status, /backup create, /backup maintenance, committed repository HEAD and /backup verify deep PASS.
• Preserve BDS and worker shutdown diagnostic evidence; explicitly terminate the intentionally persistent EndKeep worker following clean Windows BDS exit and require no remaining tracked child processes.

Candidate plugin change: ReallocAll/endkeep#20. Run evidence is available under the new EndKeep Python 3.15 real BDS workflow. All tests are on disposable worlds; no production server is touched.