Add RustChain Mempool Vault arena map

[Asti1982]

Summary

• Adds a new RustChain/Xonotic arena map: rustchain_mempool_vault.
• Includes the required pk3_build/maps deliverables: source .map, compiled .bsp, .mapinfo, 512x384 .tga levelshot, README, and CC-BY-SA-4.0 license.
• Adds a deterministic generator so the map source and preview can be regenerated.

Build / verification

• Generated assets with python tools/generate_rustchain_mempool_vault.py.
• Compiled locally with NetRadiant q3map2 1.5.0 / q3map2 2.5.17n:
  • BSP exit 0
  • VIS exit 0
  • LIGHT exit 0
• q3map2 log scan: no Entity in solid, no ERROR:, no leak/leaked messages.
• python -m pytest tests/test_rustchain_mempool_vault_map.py -q -> 3 passed.
• python -m pytest -q -> 48 passed, 2 skipped.
• git diff --cached --check passed before commit.

Bounty reference: Scottcjn/rustchain-bounties#14014.