Clarify and fix State's EOF behavior

gigaparsec.go

@@ -7,6 +7,7 @@ import (
 	"errors"
 	"fmt"
 	"io"
+	"math"
 	"slices"
 	"strings"
 )
@@ -142,10 +143,19 @@ type State[In any] struct {
 }
 
 func (s State[In]) Read(dst []In) (n uint64, next State[In], err error) {
+	if s.pos > math.MaxInt64 {
+		return 0, s, io.EOF
+	}
 	nread, err := s.r.ReadAt(dst, int64(s.pos))
 	if nread > 0 {
 		s.pos += uint64(nread)
 	}
+	if nread == len(dst) && err == io.EOF {
+		if nread == 0 {
+			return 0, s, io.EOF
+		}
+		return uint64(nread), s, nil
+	}
 	return uint64(nread), s, err
 }