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/authorship.test.ts

@@ -16,10 +16,12 @@ async function getApi(): Promise<CowritingApi> {
 
 // F10 host E2E (no LLM): the rewrite of the obsolete F9 authorship-mode test.
 // F9's "authorship" mode / renderAuthorship is gone — the on-state renderReview
-// now author-colors Claude's PENDING proposal blocks as cw-ins-claude. After the
-// proposal is accepted the baseline advances (INV-18/F12) and the text becomes
-// "unchanged" → renders plain. The class to check is cw-ins-claude (proposal block),
-// NOT cw-by-claude (the old F9 authorship render). Owns its own markdown doc.
+// now author-colors Claude's PENDING proposal blocks as cw-ins-claude. This
+// suite checks that render BEFORE accepting (the class to check is cw-ins-claude,
+// the proposal block, NOT cw-by-claude — the old F9 authorship render) and that
+// the attribution data layer survives the accept; it does not exercise the F6
+// baseline (see diffView.test.ts / trackChangesPreview.test.ts / baselineRouter.test.ts
+// for the post-accept baseline behavior, INV-7/D21). Owns its own markdown doc.
 suite("F10 review preview — pending Claude proposal renders cw-ins-claude in the on-state (host E2E, no LLM)", () => {
   const DOC_REL = "docs/f10claude.md";
   const TARGET = "The sentence Claude will compose over.";
@@ -65,7 +67,9 @@ suite("F10 review preview — pending Claude proposal renders cw-ins-claude in t
       "pending Claude proposal block carries cw-ins-claude in the on-state render",
     );
 
-    // accept via the seam — baseline advances (machine-landing, INV-18/F12)
+    // accept via the seam — the proposal lands; the baseline no longer auto-advances
+    // (INV-18/#48 retired, spec INV-7), but that isn't asserted here (no coediting
+    // entry / baseline established for this doc — only the attribution data layer).
     assert.ok(await api.proposalController.acceptById(DOC_REL, id!), "accept applies via the seam");
     await settle();