feat(ext): add builtin C/C++ language modules

src/ext/lang_c.h

@@ -0,0 +1,20 @@
+#ifndef ST_EXT_LANG_C_H
+#define ST_EXT_LANG_C_H
+
+#include "abi.h"
+
+/*
+ * Builtin C language module entry point (called by the builtin module
+ * registry in abi.c; a future extension-discovery todo keeps this
+ * shape for user modules too).
+ *
+ * Registers over the generic ABI:
+ *   - language "c"        (detect fn + the CC/CFLAGS variables it owns)
+ *   - check kind "header" for language "c"
+ *
+ * Returns NULL on success. Every compiler-specific string lives in
+ * lang_c.c -- nothing here is known to core.
+ */
+struct st_error *st_ext_lang_c_init(struct st_ext_ctx *ctx);
+
+#endif /* ST_EXT_LANG_C_H */