lua: Lua 5.0-style arg auto-binding in vararg functions; assert-counter diagnostic +2 tests

lib/lua/test.sh

@@ -916,6 +916,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1812)
 (eval "(lua-eval-ast \"local x = 10 if false then elseif true then local x = 99 end return x\")")
 
+;; ── Lua 5.0-style `arg` table in vararg functions ─────────────
+(epoch 1820)
+(eval "(lua-eval-ast \"function f(a, ...) return arg.n end return f({}, 1, 2, 3)\")")
+(epoch 1821)
+(eval "(lua-eval-ast \"function f(a, ...) return arg[1] + arg[2] + arg[3] end return f({}, 10, 20, 30)\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1381,6 +1387,10 @@ check 1810 "if then local shadows"          '10'
 check 1811 "else local shadows"             '10'
 check 1812 "elseif local shadows"           '10'
 
+# ── Lua 5.0-style `arg` table in vararg functions ─────────────
+check 1820 "arg.n in vararg fn"             '3'
+check 1821 "arg[i] access"                  '60'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL Lua-on-SX tests passed"