render parity unit K: 40250 DDS mip levels

DXT DDS textures upload the file's own mip levels like 40250
CGraphicImageTexture::CreateDDSTexture (max(1, dwMipMapCount), level i read
at sum(dwLinearSize>>2k), Copy of dwLinearSize>>2i bytes) instead of a
regenerated chain. Non-DDS images keep the D3DX_DEFAULT full chain and mem:
textures get one level. The invented is_ui_tex no-mip rule (icon/, ui/,
locale/, <=128px) is deleted. dxt_decode_test is registered in ctest with
mip-level cases.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

extension/CMakeLists.txt

@@ -86,6 +86,10 @@ if(MT_HOST_BUILD)
 endif()
 
 if(BUILD_TESTING AND MT_HOST_BUILD)
+    add_executable(dxt_decode_test tests/dxt_decode_test.cpp src/dxt.cpp)
+    target_include_directories(dxt_decode_test PRIVATE src)
+    add_test(NAME dxt.decode COMMAND $<TARGET_FILE:dxt_decode_test>)
+
     add_executable(net_cipher_test tests/net_cipher_test.cpp)
     target_link_libraries(net_cipher_test PRIVATE mtnet)
     add_test(NAME net.cipher_roundtrip COMMAND $<TARGET_FILE:net_cipher_test>)