maps: add RustChain Checkpoint Spire arena

[gchahal1982]

Summary

Adds a new original Xonotic RustChain arena map for bounty #14014: RustChain Checkpoint Spire.

Deliverables:

• pk3_build/maps/rustchain_checkpoint_spire.map
• pk3_build/maps/rustchain_checkpoint_spire.bsp
• pk3_build/maps/rustchain_checkpoint_spire.mapinfo
• pk3_build/maps/rustchain_checkpoint_spire.tga levelshot, 512x384
• pk3_build/maps/rustchain_checkpoint_spire.README.md
• pk3_build/maps/rustchain_checkpoint_spire.LICENSE
• deterministic generator: tools/generate_rustchain_checkpoint_spire.py
• focused package test: tests/test_rustchain_checkpoint_spire_map.py

Design

Checkpoint Spire is a compact cross-flow DM/CA arena centered on a glowing checkpoint tower, four raised validation decks, stair-fed side routes, and low hash-block cover. It is distinct from the visible Mempool Vault and Antiquity Vault submissions: this map emphasizes a central vertical reward pillar with four fast re-entry routes rather than a vault-room or antiquity-hardware showcase layout.

Verification

• python3 -B -m py_compile tools/generate_rustchain_checkpoint_spire.py
• python3 tools/generate_rustchain_checkpoint_spire.py
• NetRadiant q3map2 2.5.17n-git-b4b295d in Ubuntu 24.04 container:
  • BSP stage exit 0
  • VIS stage exit 0
  • LIGHT stage exit 0
  • log scan found no ERROR, no Entity in solid, and no leak/leaked markers
• python3 -B -m pytest tests/test_rustchain_checkpoint_spire_map.py -q -> 3 passed
• python3 -B -m pytest -q -> 48 passed, 2 skipped
• git diff --check -> clean