feat(assets): read retail assets through the vendored libra3assets library

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.

third_party/libra3assets/tests/test_main.cpp

@@ -0,0 +1,5 @@
+#include "test_main.hpp"
+
+int main() {
+    return ra3test::run_all();
+}