lua: string metatable, high-byte chars, multi-return truthy, perf

lib/lua/test.sh

@@ -1008,6 +1008,12 @@ cat > "$TMPFILE" << 'EPOCHS'
 (epoch 1925)
 (eval "(lua-eval-ast \"return string.format('%.3s', 'hello')\")")
 
+;; ── New-style vararg: arg is nil when ... used in body ──────
+(epoch 1930)
+(eval "(lua-eval-ast \"function f(...) local x = {...} return arg == nil and 1 or 0 end return f(1,2,3)\")")
+(epoch 1931)
+(eval "(lua-eval-ast \"function f(...) return select('#', ...) end return f(10,20,30)\")")
+
 EPOCHS
 
 OUTPUT=$(timeout 60 "$SX_SERVER" < "$TMPFILE" 2>/dev/null)
@@ -1530,6 +1536,10 @@ check 1923 "%X HEX"                         '"FF"'
 check 1924 "%c char"                        '"A"'
 check 1925 "%.3s precision"                 '"hel"'
 
+# ── New-style vararg: arg is nil when ... used in body ───────
+check 1930 "new-style vararg arg==nil"      '1'
+check 1931 "select # vararg count"          '3'
+
 TOTAL=$((PASS + FAIL))
 if [ $FAIL -eq 0 ]; then
   echo "ok $PASS/$TOTAL Lua-on-SX tests passed"