feat(f7.1): sequence-diagram parser (#22)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

test/mermaidSequenceDiff.test.ts

@@ -0,0 +1,24 @@
+import { describe, it, expect } from "vitest";
+import { parseSequence } from "../src/mermaidSequenceDiff";
+
+describe("parseSequence", () => {
+  it("captures explicit participants in order", () => {
+    const d = parseSequence("sequenceDiagram\n participant A\n participant B\n A->>B: hi");
+    expect(d.participants).toEqual(["A", "B"]);
+  });
+
+  it("captures message statements verbatim (trimmed)", () => {
+    const d = parseSequence("sequenceDiagram\n A->>B: hi\n B-->>A: bye");
+    expect(d.statements).toEqual(["A->>B: hi", "B-->>A: bye"]);
+  });
+
+  it("infers participants from messages when not declared", () => {
+    const d = parseSequence("sequenceDiagram\n A->>B: hi\n B->>C: on");
+    expect(d.participants).toEqual(["A", "B", "C"]);
+  });
+
+  it("ignores comments and blank lines", () => {
+    const d = parseSequence("sequenceDiagram\n%% note\n\n A->>B: hi");
+    expect(d.statements).toEqual(["A->>B: hi"]);
+  });
+});