chore: import upstream snapshot with attribution

internal/cbm/preprocessor.h

@@ -0,0 +1,25 @@
+#ifndef CBM_PREPROCESSOR_H
+#define CBM_PREPROCESSOR_H
+
+#include <stddef.h>
+
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+// Preprocess C/C++ source: expand macros, evaluate #ifdef, resolve #include.
+// Returns malloc-allocated expanded source, or NULL if no expansion needed/on failure.
+// extra_defines: NULL-terminated array of "NAME=VALUE" strings (can be NULL).
+// include_paths: NULL-terminated array of directory paths for #include resolution (can be NULL).
+// The returned string must be freed with cbm_preprocess_free().
+char *cbm_preprocess(const char *source, int source_len, const char *filename,
+                     const char **extra_defines, const char **include_paths, int cpp_mode);
+
+// Free preprocessed source returned by cbm_preprocess.
+void cbm_preprocess_free(char *expanded);
+
+#ifdef __cplusplus
+}
+#endif
+
+#endif // CBM_PREPROCESSOR_H