feat: baseline router — git HEAD + snapshot per INV-7/D13/D14; retire machine-landing advance (spec §6.4)

Task 2 of the native-surfaces migration plan. GitBaselineAdapter resolves a
document's HEAD blob via the built-in vscode.git extension (hardened with a
.git/logs/HEAD watch that nudges repo.status(), since the git extension's own
watcher doesn't reliably notice out-of-band commits on non-workspace-folder
repos in the E2E host). DiffViewController is reworked into a baseline router:
head mode (git-tracked, never persisted, re-read on commit) vs snapshot mode
(captured on CoeditingRegistry entry, re-pinned by "Mark Changes as Reviewed"
— renamed from cowriting.pinDiffBaseline, D14). The shipped machine-landing
baseline advance (#48/INV-18) is retired: a landed Claude edit now stays a
visible change-since-baseline until commit or review (INV-7/D21). Legacy
on-disk reasons ("opened"/"machine-landing") migrate to "entered" via
BaselineStore.normalizeReason on load.

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

test/e2e/suite/f11Toolbar.test.ts

@@ -14,7 +14,9 @@ async function getApi(): Promise<CowritingApi> {
   return api;
 }
 
-/** Create + open a fresh markdown doc under WS, returning the doc + its uri key. */
+/** Create + open a fresh markdown doc under WS, returning the doc + its uri key.
+ *  Also enters coediting (INV-10/Task 2): the baseline is established only on
+ *  entry, not merely on open. */
 async function freshDoc(rel: string, body: string): Promise<{ doc: vscode.TextDocument; key: string }> {
   const abs = path.join(WS, rel);
   fs.mkdirSync(path.dirname(abs), { recursive: true });
@@ -22,6 +24,7 @@ async function freshDoc(rel: string, body: string): Promise<{ doc: vscode.TextDo
   const uri = vscode.Uri.file(abs);
   const doc = await vscode.workspace.openTextDocument(uri);
   await vscode.window.showTextDocument(doc);
+  await vscode.commands.executeCommand("cowriting.coeditDocument");
   await settle();
   return { doc, key: uri.toString() };
 }
@@ -57,12 +60,13 @@ suite("F11 preview toolbar (host E2E — message → seam wiring, no LLM)", () =
   });
 
   // SLICE-1 reachability: the orphaned pin command gets a real palette `when`.
-  test("pinDiffBaseline is reachable from the command palette (when: editorLangId == markdown)", async () => {
+  // Renamed cowriting.pinDiffBaseline → cowriting.markReviewed (Task 2, D14).
+  test("markReviewed is reachable from the command palette (when: editorLangId == markdown)", async () => {
     const pkg = JSON.parse(fs.readFileSync(path.join(__dirname, "../../../../package.json"), "utf8"));
     const entry = (pkg.contributes.menus.commandPalette as Array<{ command: string; when?: string }>).find(
-      (m) => m.command === "cowriting.pinDiffBaseline",
+      (m) => m.command === "cowriting.markReviewed",
     );
-    assert.ok(entry, "pinDiffBaseline has a commandPalette entry");
+    assert.ok(entry, "markReviewed has a commandPalette entry");
     assert.notStrictEqual(entry!.when, "false", "it is no longer hidden (when:false)");
     assert.match(entry!.when ?? "", /editorLangId == markdown/, "guarded on markdown");
   });