feat: add comprehensive error handling with typed error codes

Add strict error handling for stk initialization and platform operations.
stk_init() now fails fast on critical errors and returns typed error codes
that users can handle programmatically.

Changes:
- Add STK_INIT_* error codes for init failures (memory, tmpdir, watch)
- Add STK_PLATFORM_* error codes for platform operation results
- Check platform_mkdir() and fail if temp directory cannot be created
- Check platform_directory_watch_start() and fail if watch cannot initialize
- Add error_cleanup path in platform_directory_watch_start() to properly
  free resources on critical Windows failures.
- Replace all platform error checks with STK_PLATFORM_OPERATION_SUCCESS
- Log FATAL errors with when critical operations fail
- Add warning logs for non-critical failures

Critical failures now return specific error codes:
- STK_INIT_MEMORY_ERROR: Module memory allocation failed
- STK_INIT_TMPDIR_ERROR: Cannot create temp directory
- STK_INIT_WATCH_ERROR: Cannot start directory watching

Individual module load failures remain non-fatal and are handled logged.

include/stk.h

@@ -10,12 +10,25 @@
 #define STK_PATH_MAX 256
 #define STK_PATH_MAX_OS 4096
 
-/* Modules */
+/* Initialization return codes */
+#define STK_INIT_SUCCESS 0
+#define STK_INIT_MEMORY_ERROR 1
+#define STK_INIT_TMPDIR_ERROR 2
+#define STK_INIT_WATCH_ERROR 3
+
+/* Module loading return codes */
 #define STK_MOD_INIT_SUCCESS 0
 #define STK_MOD_INIT_FAILURE 1
 #define STK_MOD_LIBRARY_LOAD_ERROR 2
 #define STK_MOD_SYMBOL_NOT_FOUND_ERROR 3
 
+/* Platform return codes */
+#define STK_PLATFORM_OPERATION_SUCCESS 0
+#define STK_PLATFORM_FILE_COPY_ERROR 1
+#define STK_PLATFORM_MKDIR_ERROR 2
+#define STK_PLATFORM_REMOVE_DIR_ERROR 3
+#define STK_PLATFORM_REMOVE_FILE_ERROR 4
+
 #if defined(__linux__) || defined(_WIN32)
 #define STK_EVENT_BUFFER 4096
 #endif