Replace upvalue-based shell dispatch with OP_LUSH opcode

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.

ljumptab.h

@@ -106,6 +106,7 @@ static const void *const disptab[NUM_OPCODES] = {
 &&L_OP_SETLIST,
 &&L_OP_CLOSURE,
 &&L_OP_VARARG,
+&&L_OP_LUSH,
 &&L_OP_GETVARG,
 &&L_OP_ERRNNIL,
 &&L_OP_VARARGPREP,