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/trackChangesPreview.test.ts

@@ -7,10 +7,14 @@ const WS = process.env.E2E_WORKSPACE!;
 const DOC_REL = "docs/preview.md";
 const docUri = () => vscode.Uri.file(path.join(WS, DOC_REL)).toString();
 
+/** Open the doc AND enter coediting (INV-10/Task 2) — the baseline is
+ *  established only on entry; re-entering an already-coedited doc is a no-op
+ *  so the once-captured baseline survives across this order-dependent suite. */
 async function openDoc(rel = DOC_REL): Promise<vscode.TextDocument> {
   const uri = vscode.Uri.file(path.join(WS, rel));
   const doc = await vscode.workspace.openTextDocument(uri);
   await vscode.window.showTextDocument(doc);
+  await vscode.commands.executeCommand("cowriting.coeditDocument");
   return doc;
 }
 async function getApi(): Promise<CowritingApi> {
@@ -58,7 +62,7 @@ suite("F7 track-changes preview (host E2E — markdown only, programmatic seam,
     );
   });
 
-  test("accepting a proposal advances the baseline; the landed block goes unchanged (PUC-3, INV-18)", async () => {
+  test("accepting a proposal does NOT advance the baseline; the landed block stays marked (PUC-3, INV-7/D21, #48 retired)", async () => {
     const doc = await openDoc();
     const api = await getApi();
     const start = doc.getText().indexOf(TARGET);
@@ -75,23 +79,24 @@ suite("F7 track-changes preview (host E2E — markdown only, programmatic seam,
     assert.ok(id, "propose returns an id");
     assert.ok(await api.proposalController.acceptById(DOC_REL, id!), "accept applies via the seam");
     await settle();
-    // The baseline advanced to include REPLACEMENT, so the buffer == baseline for
-    // that block → it is NOT marked. Confirm the replacement is not flagged as a change.
+    // Native-surfaces migration (INV-7/D21): the shipped machine-landing baseline
+    // advance (#48/INV-18) is retired — the landed block stays a visible change
+    // until "Mark Changes as Reviewed".
     const model = api.trackChangesPreviewController.getLastModel(docUri()) ?? [];
     const replacementMarked = model.some(
       (o) => o.kind !== "unchanged" && o.block.raw.includes("CLAUDE REWROTE"),
     );
-    assert.ok(!replacementMarked, "the just-landed text renders unmarked (baseline advanced)");
+    assert.ok(replacementMarked, "the just-landed text still renders marked (baseline does not auto-advance)");
   });
 
-  test("pin resets the baseline to now → preview shows no marks (PUC-4)", async () => {
+  test("markReviewed resets the baseline to now → preview shows no marks (PUC-4)", async () => {
     const doc = await openDoc();
     const api = await getApi();
-    await vscode.commands.executeCommand("cowriting.pinDiffBaseline");
+    await vscode.commands.executeCommand("cowriting.markReviewed");
     await settle();
     assert.ok(
       kinds(api).every((k) => k === "unchanged"),
-      "after pin, every block is unchanged (baseline == buffer)",
+      "after markReviewed, every block is unchanged (baseline == buffer)",
     );
     void doc;
   });
@@ -112,7 +117,7 @@ suite("F7 track-changes preview (host E2E — markdown only, programmatic seam,
     // Show the preview and pin the baseline so the fixture's `a --> b` flowchart
     // is the "before"; then add an edge `a --> c` and confirm the intra-diagram diff.
     await vscode.commands.executeCommand("cowriting.showTrackChangesPreview");
-    await vscode.commands.executeCommand("cowriting.pinDiffBaseline");
+    await vscode.commands.executeCommand("cowriting.markReviewed");
     await settle();
 
     const anchor = doc.getText().indexOf("a --> b");