Vendor libra3assets (C++26 modules: BIG4, RefPack, BinaryAsset, CSF, CkMp map)
under third_party/ and add the ra3_assets target. ra3.fs, ra3.map and ra3.terrain
become thin adapters over it:
- ra3.fs delegates RefPack and the BIG4 index/payload reads (index-only, payloads
read on demand).
- ra3.map decodes ObjectsList and CSF via map_document/csf_table; starts come from
player_starts(), replacing the off-by-one whole-buffer scan.
- ra3.terrain takes the CkMp container and HeightMapData from map_document, with
BlendTileData (not modelled by the library) parsed from the chunk payload.
Adds an opt-in real-asset check (OPENRA3_TEST_ASSETS) plus the library's own unit
suite as ra3assets_unit.
EnderTheCoder