fixed sync alignment with unequal elements

fixes #21

tests/sequence/sync/.gitignore

@@ -0,0 +1,4 @@
+# generated by tytanic, do not edit
+
+diff/**
+out/**

tests/sequence/sync/test.typ

@@ -0,0 +1,37 @@
+#set page(width: auto, height: auto)
+#import "/src/lib.typ": *
+
+#diagram({
+  _par("a")
+  _par("b")
+  _par("c")
+  _sync({
+    _seq("b", "a")
+    _seq("b", "c")
+  })
+  _gap()
+  _sync({
+    _seq("b", "a", comment: [Comment])
+    _seq("b", "c")
+  })
+  _gap()
+  _sync({
+    _seq("b", "a")
+    _seq("b", "c", comment: [Comment])
+  })
+  _gap()
+  _sync({
+    _seq("b", "a", comment: [Two\ lines])
+    _seq("b", "c", comment: [Comment])
+  })
+
+  _sync({
+    _seq("b", "a")
+    _seq("b", "c", slant: 10)
+  })
+
+  _sync({
+    _seq("b", "a")
+    _seq("b", "b")
+  })
+})