refactor: centralize STB implementation and add Emscripten support

  Description:
   - Centralized STB implementations: Moved #define STB_IMAGE_IMPLEMENTATION and
     related defines from individual command files to a new src/core/stb_impl.cpp to
     prevent multiple definition errors and improve compilation times.
   - Added Emscripten (WASM) support:
       - Forced single-threaded execution in spratlayout, spratpack, and spratunpack
         when compiled with __EMSCRIPTEN__.
       - Refactored spratlayout packing loops into reusable worker functions
         (run_guided_worker, run_shelf_worker) to support both synchronous and
         multi-threaded execution paths.
   - CMake improvements:
       - Integrated command entry points and stb_impl.cpp into the spratcore static
         library.
       - Enhanced LibArchive discovery to prefer targets and handle include
         directories more robustly.
       - Made test building conditional on the BUILD_TESTING option.

Author: pedroac

CMakeLists.txt

@@ -123,7 +123,16 @@ add_library(spratcore STATIC
     src/core/layout_parser.cpp
     src/core/output_pattern.cpp
     src/core/i18n.cpp
+    src/core/stb_impl.cpp
+    src/commands/spratlayout_command.cpp
+    src/commands/spratpack_command.cpp
+    src/commands/spratconvert_command.cpp
+    src/commands/spratframes_command.cpp
+    src/commands/spratunpack_command.cpp
 )
+target_include_directories(spratcore PUBLIC src)
+target_include_directories(spratcore SYSTEM PRIVATE ${STB_DIR})
+target_include_directories(spratcore PRIVATE ${LIBARCHIVE_INCLUDE_DIRS})
 
 option(SPRAT_ENABLE_NLS "Enable gettext-based translations when available" ON)
 option(SPRAT_REQUIRE_GETTEXT "Fail configure if gettext support is requested but unavailable" OFF)
@@ -172,7 +181,13 @@ target_compile_definitions(spratcore PUBLIC SPRAT_LOCALE_DIR="${CMAKE_INSTALL_FU
 # Binaries
 # For Windows cross-compilation, PkgConfig often leaks host paths. 
 # We prefer find_package or explicit paths.
-if(WIN32 OR CMAKE_SYSTEM_NAME STREQUAL "Windows")
+if(TARGET LibArchive::LibArchive)
+    set(LIBARCHIVE_LIBRARIES LibArchive::LibArchive)
+    set(LIBARCHIVE_FOUND TRUE)
+    if(NOT LIBARCHIVE_INCLUDE_DIRS)
+        get_target_property(LIBARCHIVE_INCLUDE_DIRS LibArchive::LibArchive INTERFACE_INCLUDE_DIRECTORIES)
+    endif()
+elseif(WIN32 OR CMAKE_SYSTEM_NAME STREQUAL "Windows" OR EMSCRIPTEN)
     find_package(LibArchive QUIET)
     if(LibArchive_FOUND)
         set(LIBARCHIVE_LIBRARIES LibArchive::LibArchive)
@@ -329,5 +344,7 @@ install(DIRECTORY transforms/
 install(FILES man/sprat-cli.1
     DESTINATION ${CMAKE_INSTALL_MANDIR}/man1)
 
-enable_testing()
-add_subdirectory(tests)
+if(BUILD_TESTING)
+    enable_testing()
+    add_subdirectory(tests)
+endif()

src/commands/spratframes_command.cpp

@@ -40,7 +40,6 @@ namespace fs = std::filesystem;
 #include "core/cli_parse.h"
 #include "core/i18n.h"
 
-#define STB_IMAGE_IMPLEMENTATION
 #include <stb_image.h>
 
 // Configuration