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/trackChangesModel.test.ts

@@ -440,9 +440,9 @@ describe("renderReview", () => {
     expect(html).toContain('data-proposal-id="p1"'); // proposal still shows (it is an action)
     expect(html).toContain("Person");
   });
-  test("renderReview: zero diff (e.g. machine-landing accept) renders unchanged blocks plain (Task 2 behavior)", () => {
+  test("renderReview: zero diff (baseline == current) renders unchanged blocks plain (Task 2 behavior)", () => {
     // Task 2: unchanged blocks always render plain — color means "changed since baseline".
-    // After accepting a Claude edit the text is the new baseline; no diff → no coloring.
+    // When baseline == current (e.g. right after a fresh snapshot) there's no diff → no coloring.
     const doc = "Human wrote this.\n\nClaude wrote that.";
     const spans: AuthorSpan[] = [{ start: 19, end: doc.length, author: "claude" }];
     const html = renderReview(doc, doc, spans, []); // no pinned flag