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.

lcmd.h

@@ -9,13 +9,12 @@
 
 #include "lua.h"
 
-/* Upvalue indices for shell functions on the main chunk.
-** Index 0 is _ENV (standard). Indices 1-4 are lush shell functions. */
-#define LUSH_UPVAL_COMMAND      1
-#define LUSH_UPVAL_INTERACTIVE  2
-#define LUSH_UPVAL_GETENV       3
-#define LUSH_UPVAL_SETENV       4
-#define LUSH_NUM_UPVALS         5  /* total: _ENV(0) + 4 shell functions */
+/* OP_LUSH sub-operation indices (B operand selects which function) */
+#define LUSH_OP_COMMAND      0
+#define LUSH_OP_INTERACTIVE  1
+#define LUSH_OP_GETENV       2
+#define LUSH_OP_SETENV       3
+#define LUSH_OP_COUNT        4
 
 int lushCmd_command (lua_State *L);
 int lushCmd_interactive (lua_State *L);