write tool fixes

main.cpp

@@ -229,6 +229,8 @@ static int AL(Cfg &cfg, Messages &msgs, Usage &out, volatile int *interrupted, s
           else if (!has_line && !has_byte && !has_del_lines && !has_del_bytes) {
             /* No offset/deletion parameters: overwrite the whole file. */
             std::string c = filter_text(cv);
+            /* Ensure the written file ends with a single trailing newline. */
+            if (!c.empty() && c.back() != '\n') c.push_back('\n');
             auto sl = fp.find_last_of('/');
             if (sl != std::string::npos) mkdir_p(std::string_view(fp).substr(0, sl));
             else mkdir_p(".");
@@ -236,6 +238,10 @@ static int AL(Cfg &cfg, Messages &msgs, Usage &out, volatile int *interrupted, s
             else { res = sfmt("Successfully wrote %zu bytes to %s", c.size(), fp.c_str()); }
           } else {
             int werr = 0; std::string c = filter_text(cv);
+            /* Line-based edits keep the text-file trailing-newline convention;
+             * byte-based edits are exact byte operations and must not have a
+             * newline injected. */
+            if (!(has_byte || has_del_bytes) && !c.empty() && c.back() != '\n') c.push_back('\n');
             res = write_file(fp, line_off, byte_off, del_lines, del_bytes, c, &werr);
             if (werr) sty = 31;
           }