add parameter expansion, arithmetic, set -e/-x/pipefail, trap, eval, exec

- ${#v}, ${v#p} ${v##p} ${v%p} ${v%%p}, ${v/p/r} ${v//p/r} ${v/#p/r}
  ${v/%p/r}, ${v:off:len}, ${v-w} ${v:=w} ${v:+w} ${v:?msg}; nested ${}
- arithmetic expansion $((...)) with the C operators, ** and assignments
- set -e, set -x, set -o pipefail (also as gosh options, passed on to
  subshells); set shows variables, set -o the options
- trap for EXIT and signals; scripts with an EXIT trap run it on
  SIGINT/SIGTERM/SIGHUP too
- eval, exec (also exec > file without a command), $!, wait pid

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

subst.go

@@ -79,16 +79,14 @@ func subshellCommand(script string, params []string) (*exec.Cmd, error) {
 			return nil, err
 		}
 	}
-	args := append([]string{"-c", functionDefs() + script, scriptName}, params...)
+	args := append(optionFlags(), "-c", functionDefs()+script, scriptName)
+	args = append(args, params...)
 	return exec.Command(path, args...), nil
 }
 
 // commandSubst runs command in a subshell and returns its output without
 // trailing newlines. stdin and stderr are those of the shell.
 func commandSubst(command string) (string, error) {
-	if strings.HasPrefix(command, "(") {
-		return "", errors.New("arithmetic expansion $((...)) is not supported")
-	}
 	var out bytes.Buffer
 	cmd, err := subshellCommand(command, positional)
 	if err != nil {