feat(f7.1): mermaid diff dispatcher + type detection (#22)

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

test/mermaidDiff.test.ts

@@ -0,0 +1,29 @@
+import { describe, it, expect } from "vitest";
+import { detectDiagramType, diffMermaid } from "../src/mermaidDiff";
+
+describe("detectDiagramType", () => {
+  it("detects flowchart from `flowchart` / `graph` headers", () => {
+    expect(detectDiagramType("flowchart LR\n a-->b")).toBe("flowchart");
+    expect(detectDiagramType("graph TD\n a-->b")).toBe("flowchart");
+    expect(detectDiagramType("  \n%% c\ngraph TD")).toBe("flowchart");
+  });
+  it("detects sequence from `sequenceDiagram`", () => {
+    expect(detectDiagramType("sequenceDiagram\n A->>B: hi")).toBe("sequence");
+  });
+  it("returns `other` for unsupported types", () => {
+    expect(detectDiagramType("classDiagram\n class A")).toBe("other");
+    expect(detectDiagramType("stateDiagram-v2\n s1 --> s2")).toBe("other");
+    expect(detectDiagramType("")).toBe("other");
+  });
+});
+
+describe("diffMermaid", () => {
+  it("falls back for an unsupported diagram type", () => {
+    expect(diffMermaid("classDiagram\n class A", "classDiagram\n class B")).toEqual({ kind: "fallback" });
+  });
+  it("falls back (never throws) when the differ throws", () => {
+    // A deliberately malformed flowchart still degrades gracefully.
+    const r = diffMermaid("flowchart LR", "flowchart LR\n a -->");
+    expect(["augmented", "fallback"]).toContain(r.kind);
+  });
+});