Handle very large BigInt literals with negation in EXPLAIN output

Large integers stored as BigInt are converted to Float64 in scientific
notation when negated, matching ClickHouse's EXPLAIN AST behavior.

Author: claude

internal/explain/expressions.go

@@ -2,6 +2,7 @@ package explain
 
 import (
 	"fmt"
+	"strconv"
 	"strings"
 
 	"github.com/sqlc-dev/doubleclick/ast"
@@ -434,6 +435,19 @@ func explainUnaryExpr(sb *strings.Builder, n *ast.UnaryExpr, indent string, dept
 				s := FormatFloat(-val)
 				fmt.Fprintf(sb, "%sLiteral Float64_%s\n", indent, s)
 				return
+			case ast.LiteralString:
+				// Handle BigInt - very large numbers stored as strings
+				// ClickHouse converts these to Float64 in scientific notation
+				if lit.IsBigInt {
+					if strVal, ok := lit.Value.(string); ok {
+						// Parse the string as float64 and negate it
+						if f, err := strconv.ParseFloat(strVal, 64); err == nil {
+							s := FormatFloat(-f)
+							fmt.Fprintf(sb, "%sLiteral Float64_%s\n", indent, s)
+							return
+						}
+					}
+				}
 			}
 		}
 	}

parser/testdata/01666_gcd_ubsan/metadata.json

@@ -1,7 +1 @@
-{
-  "explain_todo": {
-    "stmt6": true,
-    "stmt7": true,
-    "stmt8": true
-  }
-}
+{}

parser/testdata/01666_lcm_ubsan/metadata.json

@@ -1,7 +1 @@
-{
-  "explain_todo": {
-    "stmt6": true,
-    "stmt7": true,
-    "stmt8": true
-  }
-}
+{}

parser/testdata/03667_accurate_cast_datetime_overflow/metadata.json

@@ -1,6 +1,5 @@
 {
   "explain_todo": {
-    "stmt2": true,
-    "stmt3": true
+    "stmt2": true
   }
 }