Compile backtick commands as __command() calls with ${} interpolation

Backtick expressions now compile as calls to a global __command()
function instead of being treated as plain strings. Interpolation
via ${expr} is supported, with each interpolated value wrapped in
tostring() for type safety. Commands are parsed in primaryexp so
suffix operations like `.stdout` work directly.

Adds a stub __command that returns {code, stdout, stderr} for testing
until the real implementation (issue #03).

linit.c

@@ -40,6 +40,33 @@ static const luaL_Reg stdlibs[] = {
 };
 
 
+/*
+** Stub __command function for backtick command syntax.
+** Returns a table {code=0, stdout=cmdstring, stderr=""}.
+** The real implementation will be provided by issue #03.
+*/
+static int luaB_command (lua_State *L) {
+  const char *cmd = luaL_checkstring(L, 1);
+  lua_createtable(L, 0, 3);
+  lua_pushinteger(L, 0);
+  lua_setfield(L, -2, "code");
+  lua_pushstring(L, cmd);
+  lua_setfield(L, -2, "stdout");
+  lua_pushliteral(L, "");
+  lua_setfield(L, -2, "stderr");
+  return 1;
+}
+
+
+/*
+** Register shell built-in globals (called after standard libraries).
+*/
+static void opencommand (lua_State *L) {
+  lua_pushcfunction(L, luaB_command);
+  lua_setglobal(L, "__command");
+}
+
+
 /*
 ** require and preload selected standard libraries
 */
@@ -59,5 +86,6 @@ LUALIB_API void luaL_openselectedlibs (lua_State *L, int load, int preload) {
   }
   lua_assert((mask >> 1) == LUA_UTF8LIBK);
   lua_pop(L, 1);  /* remove PRELOAD table */
+  opencommand(L);  /* register __command global */
 }