Store __command, __interactive, __getenv, __setenv as upvalues
populated by lua_load() from the registry, keeping them invisible
to user code while accessible to the parser's codegen.
Add !command syntax that runs commands with inherited terminal (no
capture). The lexer treats ! as a statement-starting token, reading
to end-of-line with the same interpolation/escape/pipe support as
backtick commands. The C function sets _ = {code=N, stdout="", stderr=""}.
Implements luaB_command: fork/execvp with pipe-captured stdout/stderr,
argv parsing with quoting/escaping, and non-blocking pipe reading via
select(). Backs the __command() calls emitted by the backtick parser.
Resolves#03 and #04.
