feat(f7): block splitter for the render engine (#21)

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

test/trackChangesModel.test.ts

@@ -0,0 +1,29 @@
+import { describe, it, expect } from "vitest";
+import { splitBlocks } from "../src/trackChangesModel";
+
+describe("splitBlocks", () => {
+  it("splits prose paragraphs on blank lines, dropping empties", () => {
+    const blocks = splitBlocks("# Heading\n\nFirst para.\n\nSecond para.\n");
+    expect(blocks.map((b) => b.type)).toEqual(["prose", "prose", "prose"]);
+    expect(blocks[0].raw).toBe("# Heading");
+    expect(blocks[2].raw).toBe("Second para.");
+  });
+
+  it("keeps a fenced code block whole and tags it `code`", () => {
+    const text = "Intro.\n\n```ts\nconst a = 1;\n\nconst b = 2;\n```\n\nOutro.\n";
+    const blocks = splitBlocks(text);
+    expect(blocks.map((b) => b.type)).toEqual(["prose", "code", "prose"]);
+    expect(blocks[1].raw).toBe("```ts\nconst a = 1;\n\nconst b = 2;\n```");
+  });
+
+  it("tags a mermaid fence `mermaid`", () => {
+    const blocks = splitBlocks("```mermaid\nflowchart LR\n  a-->b\n```\n");
+    expect(blocks).toHaveLength(1);
+    expect(blocks[0].type).toBe("mermaid");
+  });
+
+  it("normalizes the key (whitespace-collapsed) for matching", () => {
+    const blocks = splitBlocks("Hello    world\n");
+    expect(blocks[0].key).toBe("hello world");
+  });
+});