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_string.go

@@ -15,6 +15,20 @@ type StringKSUID struct {
 	ksuid.KSUID
 }
 
+// NilStringKSUID is the zero/nil StringKSUID value.
+var NilStringKSUID = StringKSUID{KSUID: ksuid.Nil}
+
+// Equal reports whether two StringKSUID values represent the same identifier,
+// treating two nil values as equal. Implements [ApplyInterface].
+func (s StringKSUID) Equal(rh StringKSUID) bool {
+	return equalKSUID(s.KSUID, rh.KSUID)
+}
+
+// IsZero reports whether s is nil. Implements [ApplyInterface].
+func (s StringKSUID) IsZero() bool {
+	return s.IsNil()
+}
+
 // NewStringKSUID generates a new StringKSUID.
 func NewStringKSUID() StringKSUID {
 	return StringKSUID{KSUID: ksuid.New()}