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/undoMarks.test.ts

@@ -15,6 +15,9 @@ async function getApi(): Promise<CowritingApi> {
   return api;
 }
 
+/** Also enters coediting (INV-10/Task 2): the baseline is established only on
+ *  entry, not merely on open — needed for the preview's added/changed marks to
+ *  reflect a real diff rather than a vacuous baseline==current fallback. */
 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 +25,7 @@ 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() };
 }