maps: add RustChain antiquity vault arena

[eldwin-easynet-world]

Summary

• Add original rustchain_antiquity_vault Xonotic/RustChain arena map for bounty #14014.
• Include source .map, compiled .bsp, .mapinfo, 512x384+ TGA levelshot, README, CC-BY-SA-compatible license, deterministic generator, and focused delivery test.
• Layout is a distinct two-level antiquity-hardware vault with crossed ledger bridges, offset stair routes, four archive alcoves, and a central reward pedestal.

Verification

• Generated assets with python3 tools/generate_rustchain_antiquity_vault.py.
• Compiled in Ubuntu 24.04 Kubernetes pod with NetRadiant q3map2 2.5.17n-git-d9338309e:
  • q3map2 -game xonotic -fs_basepath /work -fs_game data -bsp -meta pk3_build/maps/rustchain_antiquity_vault.map
  • q3map2 -game xonotic -fs_basepath /work -fs_game data -vis pk3_build/maps/rustchain_antiquity_vault.bsp
  • q3map2 -game xonotic -fs_basepath /work -fs_game data -light -fast -patchshadows pk3_build/maps/rustchain_antiquity_vault.bsp
• Log scan found no ERROR:, no Entity in solid, and no leak/leaked messages after the final compile.
• pytest tests/test_rustchain_antiquity_vault_map.py -q -> 3 passed.
• pytest -q with pytest+Pillow -> 31 passed, 2 skipped, 17 unrelated existing prop-asset failures from missing local pk3_build/models/props/... files in this checkout.