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>

read.go

@@ -58,7 +58,8 @@ func builtinRead(args []string, stdin *os.File) error {
 	if prompt != "" && terminal {
 		fmt.Fprint(stdio(2), prompt)
 	}
-	line, literal, eof, err := readInputLine(stdin, raw, interactiveShell && terminal)
+	// Ctrl+C or a trapped signal ends a read from the terminal
+	line, literal, eof, err := readInputLine(stdin, raw, terminal && (interactiveShell || len(trapChans) > 0))
 	if err != nil {
 		return err
 	}
@@ -160,6 +161,9 @@ func interruptibleByteReader(f *os.File, intr chan os.Signal) func() (byte, erro
 				return 0, statusError(128 + int(syscall.SIGINT))
 			default:
 			}
+			if hasPendingSignal() {
+				return 0, statusError(128 + int(syscall.SIGINT))
+			}
 			n, err := unix.Poll(fds, 100)
 			if err != nil && !errors.Is(err, unix.EINTR) {
 				return 0, err