Use sigsetjmp/siglongjmp to catch SIGINT during readline/fgets input
and jump back to the REPL loop with a fresh prompt. Running Lua code
still gets interrupted via the existing laction/lstop mechanism.
Fixes#11.
Rewrite #10 with refined interactive command design: bare-word REPL
fallback instead of ! prefix, result table assigned to _, same shape
as captured commands. Add #11 for Ctrl-C clearing the current line
instead of exiting the REPL.