codepage: replace iconv with bundled WindowsBestFit tables

mt_codepage implements code pages 874/932/936/949/950/1250-1258 from
Microsoft's bestfit<cp>.txt sources (script/gen_codepage_tables.py,
SHA-256 pinned). Win32Crt's MultiByteToWideChar/WideCharToMultiByte/
IsDBCSLeadByteEx and net/text_codec use it, so the Android API 24 build
no longer depends on iconv. port.codepage checks every table against
its source file.

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

extension/CMakeLists.txt

@@ -14,6 +14,8 @@ add_subdirectory(godot-cpp)
 # aliases mt3p::sodium / mt3p::zstd / mt3p::minilzo. See docs/THIRD-PARTY.md.
 add_subdirectory(third_party)
 
+add_subdirectory(src/codepage)
+
 # --- mtnet: Metin2 net protocol core (no godot-cpp dep; libsodium only) ---
 add_library(mtnet STATIC
     src/net/text_codec.cpp
@@ -27,13 +29,7 @@ add_library(mtnet STATIC
     src/net/classic/classic_cipher.cpp
 )
 target_include_directories(mtnet PUBLIC src/net)
-target_link_libraries(mtnet PUBLIC mt3p::sodium mt3p::minilzo mt3p::cryptopp)
-# macOS ships iconv as a system library rather than libc. Linux/glibc and
-# Windows use the platform implementation without this extra link item.
-find_library(MT_ICONV_LIBRARY NAMES iconv)
-if(MT_ICONV_LIBRARY)
-    target_link_libraries(mtnet PUBLIC ${MT_ICONV_LIBRARY})
-endif()
+target_link_libraries(mtnet PUBLIC mt3p::sodium mt3p::minilzo mt3p::cryptopp mt_codepage)
 target_compile_features(mtnet PUBLIC cxx_std_20)
 # §1.5: non-EUROPE CG_CLIENT_VERSION timestamp, in C __TIMESTAMP__ form
 # ("Www Mmm dd hh:mm:ss yyyy"); regenerated on each CMake configure.