feat: native diff surface — QuickDiff + cowriting-baseline: provider + Review Changes + status bar (spec §6.4, INV-13)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

test/e2e/suite/baselineRouter.test.ts

@@ -16,10 +16,14 @@ suite("baseline router (PUC-1, INV-7/D13/D14)", () => {
     assert.strictEqual(api.diffViewController.modeOf(doc.uri.toString()), "snapshot");
     assert.strictEqual(api.diffViewController.getBaseline(doc.uri.toString())?.text, "one\n");
     await ed.edit((b) => b.insert(new vscode.Position(1, 0), "two\n"));
+    // Task 3 (INV-13): the status-bar/SCM change count tracks the live edit.
+    await settleUntil(() => api.scmSurfaceController.changeCount(doc.uri.toString()) === 1);
     await vscode.commands.executeCommand("cowriting.markReviewed");
     const b = api.diffViewController.getBaseline(doc.uri.toString());
     assert.strictEqual(b?.text, "one\ntwo\n");
     assert.strictEqual(b?.reason, "pinned");
+    // A pin re-baselines to the clean current text, so the change count resets.
+    await settleUntil(() => api.scmSurfaceController.changeCount(doc.uri.toString()) === 0);
   });
 
   test("head mode: baseline = HEAD; commit advances it", async function () {