Tighten Seriatim input validation

internal/modules/input/seriatim/adapter.go

@@ -6,6 +6,7 @@ import (
 	"encoding/hex"
 	"fmt"
 	"math"
+	"math/big"
 	"strconv"
 	"strings"
 
@@ -124,7 +125,7 @@ func sourceUnit(segment segment, index int, seen map[string]struct{}) (source.So
 	if err != nil {
 		return source.SourceUnit{}, err
 	}
-	if end < start {
+	if end.Cmp(start) < 0 {
 		return source.SourceUnit{}, inputErrorf("segment %q end must be greater than or equal to start", segment.ID)
 	}
 
@@ -144,22 +145,26 @@ func sourceUnit(segment segment, index int, seen map[string]struct{}) (source.So
 	}, nil
 }
 
-func validTimestamp(value fmt.Stringer, label string) (float64, error) {
+func validTimestamp(value fmt.Stringer, label string) (*big.Rat, error) {
 	raw := strings.TrimSpace(value.String())
 	if raw == "" {
-		return 0, inputErrorf("%s must not be empty", label)
+		return nil, inputErrorf("%s must not be empty", label)
 	}
 	parsed, err := strconv.ParseFloat(raw, 64)
 	if err != nil {
-		return 0, inputErrorf("%s must be a valid number: %w", label, err)
+		return nil, inputErrorf("%s must be a valid number: %w", label, err)
 	}
 	if math.IsInf(parsed, 0) || math.IsNaN(parsed) {
-		return 0, inputErrorf("%s must be finite", label)
+		return nil, inputErrorf("%s must be finite", label)
 	}
 	if parsed < 0 {
-		return 0, inputErrorf("%s must not be negative", label)
+		return nil, inputErrorf("%s must not be negative", label)
 	}
-	return parsed, nil
+	rat, ok := new(big.Rat).SetString(raw)
+	if !ok {
+		return nil, inputErrorf("%s must be a valid number", label)
+	}
+	return rat, nil
 }
 
 func documentID(requestedID string, metadata map[string]any, rawDigest string) string {