test(sequence): mermaid-spec-parity forms for new arrows

Pins the exact inputs mermaid's sequenceDiagram.spec.js and cypress
rendering spec use: no-space forms (Alice-xBob:Hello Bob, how are
you?), asymmetric spacing, punctuation labels, reversed-direction
bidirectional (golden case), and loud rejection of the released
central-connection syntax (A ()->>() B) as a documented follow-up.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: cgreeno

cmd/testdata/sequence-ascii/bidirectional_arrows.txt

@@ -3,6 +3,7 @@ sequenceDiagram
     participant B
     A<<->>B: solid bidirectional
     A<<-->>B: dotted bidirectional
+    B<<->>A: reverse solid
 ---
 +---+     +---+
 | A |     | B |
@@ -14,3 +15,6 @@ sequenceDiagram
   | dotted bidirectional
   +<.......>|
   |         |
+  | reverse solid
+  |<------->+
+  |         |

cmd/testdata/sequence/bidirectional_arrows.txt

@@ -3,6 +3,7 @@ sequenceDiagram
     participant B
     A<<->>B: solid bidirectional
     A<<-->>B: dotted bidirectional
+    B<<->>A: reverse solid
 ---
 ┌───┐     ┌───┐
 │ A │     │ B │
@@ -14,3 +15,6 @@ sequenceDiagram
   │ dotted bidirectional
   ├◄┈┈┈┈┈┈┈►│
   │         │
+  │ reverse solid
+  │◄───────►┤
+  │         │

pkg/sequence/arrows_test.go

@@ -40,6 +40,55 @@ func TestArrowTypes(t *testing.T) {
 	}
 }
 
+// TestArrowSpecParityForms pins the exact input forms mermaid's own test
+// suites use for these arrows (sequenceDiagram.spec.js and the cypress
+// rendering spec): no spaces around the arrow or after the colon, punctuation
+// in labels, asymmetric spacing, and both message directions.
+func TestArrowSpecParityForms(t *testing.T) {
+	tests := []struct {
+		in   string
+		want ArrowType
+	}{
+		{"sequenceDiagram\nAlice-xBob:Hello Bob, how are you?", SolidCross},
+		{"sequenceDiagram\nAlice--xBob:Hello Bob, how are you?", DottedCross},
+		{"sequenceDiagram\nAlice-)Bob:Hello Bob, how are you?", SolidPoint},
+		{"sequenceDiagram\nAlice--)Bob:Hello Bob, how are you?", DottedPoint},
+		{"sequenceDiagram\nAlice<<->>Bob:Hello Bob, how are you?", BidirectionalSolid},
+		{"sequenceDiagram\nAlice<<-->>Bob:Hello Bob, how are you?", BidirectionalDotted},
+		{"sequenceDiagram\n Bob--x Alice: I am good thanks!", DottedCross},
+		{"sequenceDiagram\n Alice --x Ola1: Bye!", DottedCross},
+		{"sequenceDiagram\n John<<->>Alice: This also works the other way", BidirectionalSolid},
+	}
+	for _, tt := range tests {
+		sd, err := Parse(tt.in)
+		if err != nil {
+			t.Errorf("Parse(%q): %v", tt.in, err)
+			continue
+		}
+		if len(sd.Messages) != 1 || sd.Messages[0].ArrowType != tt.want {
+			t.Errorf("Parse(%q): expected 1 message with ArrowType %v, got %+v", tt.in, tt.want, sd.Messages)
+		}
+	}
+}
+
+// TestCentralConnectionsRejected documents that mermaid's central-connection
+// syntax (circle markers at the lifeline: "A ()->>() B", released in mermaid
+// 11.16) is rejected loudly rather than silently mis-parsed. It composes with
+// every arrow type, so supporting it is its own follow-up.
+func TestCentralConnectionsRejected(t *testing.T) {
+	for _, in := range []string{
+		"sequenceDiagram\n Alice ()->>() Bob: dual",
+		"sequenceDiagram\n Alice ()-x() Bob: cross dual",
+		"sequenceDiagram\n Alice ()<<->>() Bob: bidirectional dual",
+		"sequenceDiagram\n Alice ->>() Bob: forward",
+		"sequenceDiagram\n Alice ()->> Bob: reverse",
+	} {
+		if _, err := Parse(in); err == nil {
+			t.Errorf("expected error for central-connection syntax %q, got none", in)
+		}
+	}
+}
+
 // TestCrossPointBidirectionalArrows checks that mermaid's remaining message
 // arrows — cross (-x/--x), async point (-)/--)) and bidirectional
 // (<<->>/<<-->>) — parse to the right ArrowType.