feat: Task 7 — decorateCommitted fills committed author-colored changes in the editor (reverses INV-32)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test/trackChangesModel.test.ts

@@ -250,6 +250,29 @@ describe("wordDiffByAuthor", () => {
 
 import { renderPlain } from "../src/trackChangesModel";
 
+import { wordEditHunks, decorationPlan } from "../src/trackChangesModel";
+
+test("committed change: wordEditHunks(current, baseline) → start/end index current; replacement is baseline text", () => {
+  const baseline = "the light mode";
+  const current = "the dark mode";
+  const [h] = wordEditHunks(current, baseline);
+  expect(current.slice(h.start, h.end)).toContain("dark");   // applied (current) side
+  expect(h.replacement).toContain("light");                  // baseline (deleted) side
+  const plan = decorationPlan(h.start, h.replacement, current.slice(h.start, h.end));
+  expect(plan.insertions.length).toBeGreaterThan(0);
+  expect(plan.deletions.some((d) => d.text.includes("light"))).toBe(true);
+});
+
+test("committed change with NO shared prefix: current offsets + deleted text both correct", () => {
+  const baseline = "alpha tail";
+  const current = "beta tail";
+  const [h] = wordEditHunks(current, baseline);
+  expect(h.start).toBe(0);
+  expect(current.slice(h.start, h.end)).toContain("beta");   // applied
+  const plan = decorationPlan(h.start, h.replacement, current.slice(h.start, h.end));
+  expect(plan.deletions.some((d) => d.text.includes("alpha"))).toBe(true); // deletion not lost
+});
+
 describe("renderPlain", () => {
   test("renderPlain renders current buffer as plain markdown (no marks)", () => {
     const html = renderPlain("# Title\n\nhello");