runtime: actually rename str_format param to fmt

Previous commit 6d89728 had a misleading message - the rename
itself never landed (Edit-without-Read failure cascaded silently
in the parent shell). 6d89728 incidentally captured 810 lines of
in-flight work from concurrent runtime agents and shipped it under
the wrong message; the in-flight agents will land their final
verified state on top.

This commit is just the actual rename: str_format(template, data)
to str_format(fmt, data). C++ keyword conflict resolved.

el-compiler/runtime/el_runtime.c

@@ -3889,8 +3889,8 @@ el_val_t str_pad_right(el_val_t s, el_val_t width, el_val_t pad) {
     return str_pad(EL_CSTR(s), (int64_t)width, EL_CSTR(pad), 0);
 }
 
-el_val_t str_format(el_val_t template, el_val_t data) {
-    const char* tpl = EL_CSTR(template);
+el_val_t str_format(el_val_t fmt, el_val_t data) {
+    const char* tpl = EL_CSTR(fmt);
     if (!tpl) return el_wrap_str(el_strdup(""));
     JsonBuf b; jb_init(&b);
     const char* p = tpl;