move to read only memory mapped files

solanaceae/ngc_ft1_sha1/file_constructor.hpp

@@ -6,4 +6,5 @@
 #include <string_view>
 
 std::unique_ptr<File2I> construct_file2_rw_mapped(std::string_view file_path, int64_t file_size = -1);
+std::unique_ptr<File2I> construct_file2_r_mapped(std::string_view file_path);