feat(ksuid): add Equal and IsZero for ApplyInterfacePtr

StringKSUID and BinaryKSUID now implement ApplyInterface with nil-nil
equality semantics, enabling ApplyInterfacePtr in patch logic. Adds nil
sentinel vars and tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

ksuid_test.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"testing"
 
+	"github.com/go-jet/jet/v2/mysql"
 	"github.com/segmentio/ksuid"
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -118,6 +119,34 @@ func TestBinaryKSUID_Conversion(t *testing.T) {
 }
 
 
+func TestStringKSUID_EqualIsZero(t *testing.T) {
+	id := NewStringKSUID()
+	assert.True(t, NilStringKSUID.Equal(NilStringKSUID))
+	assert.True(t, id.Equal(id))
+	assert.False(t, id.Equal(NilStringKSUID))
+	assert.True(t, NilStringKSUID.IsZero())
+	assert.False(t, id.IsZero())
+}
+
+func TestBinaryKSUID_EqualIsZero(t *testing.T) {
+	id := NewBinaryKSUID()
+	assert.True(t, NilBinaryKSUID.Equal(NilBinaryKSUID))
+	assert.True(t, id.Equal(id))
+	assert.False(t, id.Equal(NilBinaryKSUID))
+	assert.True(t, NilBinaryKSUID.IsZero())
+	assert.False(t, id.IsZero())
+}
+
+func TestStringKSUID_ApplyInterfacePtr(t *testing.T) {
+	targetCol := mysql.StringColumn("id")
+	modified := make(ColumnList, 0)
+	current := Ptr(NewStringKSUID())
+	updated := ApplyInterfacePtr(current, Ptr(NewStringKSUID()), &modified, targetCol)
+	require.NotNil(t, updated)
+	assert.Len(t, modified, 1)
+	assert.False(t, current.Equal(*updated))
+}
+
 func TestParseStringKSUID(t *testing.T) {
 	id := ksuid.New()
 	parsed, err := ParseStringKSUID(id.String())