feat(f7.1): sequence participant/message diff + rect emission (#22)

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

test/mermaidSequenceDiff.test.ts

@@ -22,3 +22,44 @@ describe("parseSequence", () => {
     expect(d.statements).toEqual(["A->>B: hi"]);
   });
 });
+
+import { diffSequence } from "../src/mermaidSequenceDiff";
+
+const rectCount = (s: string, rgb: string) => s.split(`rect rgb(${rgb})`).length - 1;
+
+describe("diffSequence", () => {
+  it("wraps an added message in a green rect", () => {
+    const before = "sequenceDiagram\n A->>B: hi";
+    const current = "sequenceDiagram\n A->>B: hi\n B->>C: on";
+    const out = diffSequence(before, current);
+    expect(rectCount(out, "46, 160, 67")).toBe(1); // CW_COLORS.added as rgb
+    expect(out).toContain("B->>C: on");
+  });
+
+  it("ghosts a removed message back in a grey rect", () => {
+    const before = "sequenceDiagram\n A->>B: hi\n B->>C: gone";
+    const current = "sequenceDiagram\n A->>B: hi";
+    const out = diffSequence(before, current);
+    expect(out).toContain("B->>C: gone");
+    expect(rectCount(out, "128, 128, 128")).toBe(1);
+  });
+
+  it("re-declares a removed participant so it still appears", () => {
+    const before = "sequenceDiagram\n participant A\n participant B\n participant C\n A->>C: x";
+    const current = "sequenceDiagram\n participant A\n participant B\n A->>B: y";
+    const out = diffSequence(before, current);
+    expect(out).toMatch(/participant C/);
+  });
+
+  it("keeps an unchanged message outside any rect", () => {
+    const doc = "sequenceDiagram\n A->>B: hi";
+    const out = diffSequence(doc, doc);
+    expect(out).not.toContain("rect rgb");
+  });
+
+  it("is deterministic", () => {
+    const before = "sequenceDiagram\n A->>B: hi";
+    const current = "sequenceDiagram\n A->>B: hi\n B->>A: bye";
+    expect(diffSequence(before, current)).toBe(diffSequence(before, current));
+  });
+});