Fix parallel build race and portable mkdir in tests

1. cbm-with-ui target: add $(OBJS_VENDORED_PROD) as dependency so
   grammar .o files are compiled before linking in parallel make.
   Without this, make -jN fires the link step as soon as embed
   finishes, racing against grammar compilations.

2. test_ui.c: replace system("mkdir -p") with cbm_mkdir_p() for
   Windows compatibility. Add compat_fs.h include.

Author: DeusData

Makefile.cbm

@@ -448,7 +448,7 @@ frontend:
 embed: frontend
 	scripts/embed-frontend.sh graph-ui/dist $(BUILD_DIR)/embedded
 
-cbm-with-ui: embed
+cbm-with-ui: embed $(OBJS_VENDORED_PROD)
 	$(CC) $(CFLAGS_PROD) -o $(BUILD_DIR)/codebase-memory-mcp \
 		$(MAIN_SRC) $(PROD_SRCS_WITH_ASSETS) \
 		$(EXTRACTION_SRCS) $(AC_LZ4_SRCS) $(SQLITE_WRITER_SRC) \