VM now resolves lush functions via luaH_getshortstr through interned
TString pointers, so wrapping lush.command = my_wrapper is transparently
picked up. The lush table no longer has integer-keyed duplicates.
Register luaopen_lush() in stdlibs so the lush table is available as a
global. Users can now extend the shell by adding functions to
lush.builtins. The OP_LUSH VM access via integer keys is preserved.
Add $() for inline subcommand substitution that runs a command and
inserts its stdout (trailing newlines stripped) into the outer command
string. Supports nesting, and mixing with ${expr} and $NAME.
Add a dedicated OP_LUSH A B instruction that loads shell C functions
directly from the registry, eliminating the need for upvalue allocation
in mainfunc(), upvalue population in lua_load(), and the fragile
LUSH_NUM_UPVALS heuristic. Every chunk no longer carries 4 extra
upvalues.
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.
