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.

CMakeLists.txt

@@ -196,6 +196,25 @@ target_sources(ra3_enderlog PUBLIC FILE_SET CXX_MODULES FILES third_party/libend
 target_compile_features(ra3_enderlog PUBLIC cxx_std_26)
 openra3_target_defaults(ra3_enderlog)
 
+# --- RA3 built-in asset containers (vendored libra3assets) --------------------
+# Reader/writer for the files the retail game ships: `BIG4` archives, the EA
+# RefPack codec, SAGE `.csf` string tables and `.map` (`CkMp`) containers, plus
+# the compiled `BinaryAsset` streams. A sibling of libenderlog: C++26 modules
+# (`import ra3.assets;`) importing the standard library. It owns the on-disk
+# format knowledge, so `ra3.fs` / `ra3.map` / `ra3.terrain` become thin adapters.
+add_library(ra3_assets STATIC)
+target_sources(ra3_assets PUBLIC FILE_SET CXX_MODULES FILES
+    third_party/libra3assets/src/assets.cppm
+    third_party/libra3assets/src/error.cppm
+    third_party/libra3assets/src/bytes.cppm
+    third_party/libra3assets/src/refpack.cppm
+    third_party/libra3assets/src/big.cppm
+    third_party/libra3assets/src/binary.cppm
+    third_party/libra3assets/src/csf.cppm
+    third_party/libra3assets/src/map.cppm)
+target_compile_features(ra3_assets PUBLIC cxx_std_26)
+openra3_target_defaults(ra3_assets)
+
 # --- engine core -------------------------------------------------------------
 add_library(ra3_core STATIC)
 target_sources(ra3_core PUBLIC FILE_SET CXX_MODULES FILES src/core/ra3.core.cppm)
@@ -225,16 +244,16 @@ target_sources(ra3_game PUBLIC FILE_SET CXX_MODULES FILES src/game/ra3.game.cppm
 target_link_libraries(ra3_game PUBLIC ra3_core ra3_logic)
 openra3_target_defaults(ra3_game)
 
-# --- filesystem: BIG4 + RefPack ---------------------------------------------
+# --- filesystem: BIG4 + RefPack (adapter over libra3assets) -------------------
 add_library(ra3_fs STATIC)
 target_sources(ra3_fs PUBLIC FILE_SET CXX_MODULES FILES src/fs/ra3.fs.cppm)
-target_link_libraries(ra3_fs PUBLIC ra3_core)
+target_link_libraries(ra3_fs PUBLIC ra3_core ra3_assets)
 openra3_target_defaults(ra3_fs)
 
 # --- map discovery/loading ---------------------------------------------------
 add_library(ra3_map STATIC)
 target_sources(ra3_map PUBLIC FILE_SET CXX_MODULES FILES src/map/ra3.map.cppm)
-target_link_libraries(ra3_map PUBLIC ra3_core ra3_fs)
+target_link_libraries(ra3_map PUBLIC ra3_core ra3_fs ra3_assets)
 openra3_target_defaults(ra3_map)
 
 # --- minimal skirmish simulation --------------------------------------------
@@ -252,7 +271,7 @@ openra3_target_defaults(ra3_render)
 # --- real map terrain (HeightMapData / BlendTileData) ------------------------
 add_library(ra3_terrain STATIC)
 target_sources(ra3_terrain PUBLIC FILE_SET CXX_MODULES FILES src/terrain/ra3.terrain.cppm)
-target_link_libraries(ra3_terrain PUBLIC ra3_core ra3_fs ra3_render)
+target_link_libraries(ra3_terrain PUBLIC ra3_core ra3_fs ra3_render ra3_assets)
 openra3_target_defaults(ra3_terrain)
 
 # --- map static art (compiled W3D meshes) ------------------------------------
@@ -580,3 +599,15 @@ add_executable(ra3_tests tests/ra3_tests.cpp)
 target_link_libraries(ra3_tests PRIVATE ra3)
 openra3_target_defaults(ra3_tests)
 add_test(NAME ra3_tests COMMAND ra3_tests)
+
+# libra3assets' own dependency-free unit tests (RefPack + BIG4 + BinaryAsset +
+# CSF + CkMp round-trips), built against the vendored copy so the upstream
+# suite keeps guarding the readers OpenRA3 now relies on.
+add_executable(ra3assets_unit
+    third_party/libra3assets/tests/test_main.cpp
+    third_party/libra3assets/tests/test_assets.cpp)
+target_include_directories(ra3assets_unit PRIVATE third_party/libra3assets/tests)
+target_link_libraries(ra3assets_unit PRIVATE ra3_assets)
+target_compile_features(ra3assets_unit PRIVATE cxx_std_26)
+openra3_target_defaults(ra3assets_unit)
+add_test(NAME ra3assets_unit COMMAND ra3assets_unit)