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/coediting.test.ts

@@ -1,5 +1,6 @@
 import * as assert from "node:assert";
 import * as vscode from "vscode";
+import { activateApi, settle } from "./helpers";
 
 async function api() {
   const ext = vscode.extensions.getExtension("benstull.vscode-cowriting-plugin")!;
@@ -17,4 +18,18 @@ suite("coediting registry (PUC-7)", () => {
     await vscode.commands.executeCommand("cowriting.stopCoediting");
     assert.strictEqual(coeditingRegistry.isCoediting(doc.uri), false);
   });
+
+  // Task 3 (INV-10): a document that was never entered into coediting gets no
+  // native-diff surface at all. QuickDiff's `provideOriginalResource` isn't
+  // directly queryable from a host-E2E test, so this asserts the gate INPUTS
+  // structurally: `isCoediting` stays false and the SCM change count — which
+  // `ScmSurfaceController` only ever populates for a coediting doc — stays 0.
+  test("non-entered doc: no coediting, no change count (structural, INV-10)", async () => {
+    const api = await activateApi();
+    const doc = await vscode.workspace.openTextDocument({ language: "markdown", content: "untouched\n" });
+    await vscode.window.showTextDocument(doc);
+    await settle();
+    assert.strictEqual(api.coeditingRegistry.isCoediting(doc.uri), false);
+    assert.strictEqual(api.scmSurfaceController.changeCount(doc.uri.toString()), 0);
+  });
 });