Fix GROUPING SETS tuple unwrapping and add TOP clause support

- Fixed GROUPING SETS handling to unwrap tuple literals and output elements
  directly with correct child counts
- Added TOP clause support to SELECT parser (use MUL_PREC to stop at *)
- Added TOP clause output to EXPLAIN AST at end of SelectQuery
- Updated 3 GROUPING SETS tests that now pass

The TOP fix prevents `SELECT TOP 5 * FROM t` from being misparsed as
`TOP (5 * FROM t)` where * was treated as multiplication.

Author: claude

internal/explain/select.go

@@ -181,6 +181,10 @@ func explainSelectQuery(sb *strings.Builder, n *ast.SelectQuery, indent string,
 	if len(n.Settings) > 0 && !n.SettingsAfterFormat {
 		fmt.Fprintf(sb, "%s Set\n", indent)
 	}
+	// TOP clause is output at the end
+	if n.Top != nil {
+		Node(sb, n.Top, depth+1)
+	}
 }
 
 func explainOrderByElement(sb *strings.Builder, n *ast.OrderByElement, indent string, depth int) {
@@ -355,5 +359,9 @@ func countSelectQueryChildren(n *ast.SelectQuery) int {
 	if len(n.Settings) > 0 && !n.SettingsAfterFormat {
 		count++
 	}
+	// TOP clause
+	if n.Top != nil {
+		count++
+	}
 	return count
 }

parser/parser.go

@@ -492,7 +492,8 @@ func (p *Parser) parseSelect() *ast.SelectQuery {
 	// Handle TOP
 	if p.currentIs(token.TOP) {
 		p.nextToken()
-		sel.Top = p.parseExpression(LOWEST)
+		// Use MUL_PREC to stop at * (which would be parsed as column selector, not multiplication)
+		sel.Top = p.parseExpression(MUL_PREC)
 	}
 
 	// Parse column list

parser/testdata/01883_grouping_sets_crash/metadata.json

@@ -1,12 +1,5 @@
 {
   "explain_todo": {
-    "stmt10": true,
-    "stmt11": true,
-    "stmt12": true,
-    "stmt14": true,
-    "stmt4": true,
-    "stmt6": true,
-    "stmt8": true,
-    "stmt9": true
+    "stmt6": true
   }
 }

parser/testdata/01883_with_grouping_sets/metadata.json

@@ -1,11 +1,5 @@
 {
   "explain_todo": {
-    "stmt10": true,
-    "stmt11": true,
-    "stmt12": true,
-    "stmt6": true,
-    "stmt7": true,
-    "stmt8": true,
     "stmt9": true
   }
 }

parser/testdata/02313_group_by_modifiers_with_non_default_types/metadata.json

@@ -1,5 +1 @@
-{
-  "explain_todo": {
-    "stmt6": true
-  }
-}
+{}