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_binary.go

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