F6 #34: delete the dead two-pane diff-view UI, keep the baseline data layer

F10 (#29) made the rendered preview the single review surface and hid the F6
two-pane vscode.diff view (command + ctrl+alt+d set when:false). This removes
that now-unreachable view code:

- DiffViewController: drop toggle/findDiffTab/epochLabel/isDiffOpen, the
  `cowriting-baseline:` TextDocumentContentProvider + BASELINE_SCHEME + baselineUri
  + the content-provider change emitter, and the toggleDiffView command. The
  baseline DATA layer is fully intact — ensureBaseline/advance/pin/capture,
  getBaseline, baselineFilePath, onDidChangeBaseline, persistence (INV-19), and
  the machine-landing auto-advance (INV-18) that F7/F10 consume.
- package.json: remove the toggleDiffView command, its commandPalette entry, and
  the ctrl+alt+d keybinding.
- E2E: diffView suite keeps the baseline-data-layer tests, drops the two-pane
  view tests; the F10 + no-workspace suites assert toggleDiffView is now absent
  (was: declared-but-hidden).

Deliberate deviation from the issue's literal acceptance: pinDiffBaseline is
KEPT. The canonical Solution Design (coauthoring-interactive-review.md §6.7)
scopes the removal to the two-pane VIEW only ("keep the controller + baseline
store"); pin() lives in the baseline lifecycle (§6.4), never touches vscode.diff,
and is exercised by live F7 baseline-reset tests. Where the P3 capture draft and
the approved spec conflict, the spec wins (documentation-leads-automation).

194 unit + 49 E2E green; typecheck + build clean. No F7/F10 behavior change.

Closes #34

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/e2e/suite-no-workspace/noWorkspace.test.ts

@@ -40,8 +40,11 @@ suite("no-workspace authoring (F8 — real folder-less, #8 lineage)", () => {
       "cowriting.toggleAttribution",
       "cowriting.acceptProposal",
       "cowriting.rejectProposal",
+      // #34: the F6 two-pane diff VIEW was removed (F10 preview is the single
+      // review surface); its toggle command is gone (the baseline store stays).
+      "cowriting.toggleDiffView",
     ]) {
-      assert.ok(!all.includes(retired), `${retired} is retired (F10 preview-only)`);
+      assert.ok(!all.includes(retired), `${retired} is retired`);
     }
   });
 
@@ -73,25 +76,24 @@ suite("no-workspace authoring (F8 — real folder-less, #8 lineage)", () => {
     assert.strictEqual(api.sidecarRouter.sidecarPath(key), undefined, "untitled artifact is in-memory only (no disk)");
   });
 
-  // F6 (#19) is workspace-INDEPENDENT: its commands are real (not stubs) even
-  // with no folder open, and the toggle works on an untitled buffer.
-  test("F6 toggle works with no folder open (untitled buffer)", async () => {
+  // F6 (#19) baseline data layer is workspace-INDEPENDENT: it captures a baseline
+  // for an untitled buffer even with no folder open (the two-pane VIEW was
+  // removed in #34; only the data layer remains). pinDiffBaseline stays real.
+  test("F6 baseline data layer works with no folder open (untitled buffer)", async () => {
     const all = await vscode.commands.getCommands(true);
-    assert.ok(all.includes("cowriting.toggleDiffView"), "toggleDiffView registered");
     assert.ok(all.includes("cowriting.pinDiffBaseline"), "pinDiffBaseline registered");
+    const ext = vscode.extensions.getExtension("benstull.vscode-cowriting-plugin")!;
+    const api = (await ext.activate()) as CowritingApi;
     const untitled = await vscode.workspace.openTextDocument({ content: "no-folder scratch\n", language: "markdown" });
     await vscode.window.showTextDocument(untitled);
     await new Promise((r) => setTimeout(r, 300));
-    await vscode.commands.executeCommand("cowriting.toggleDiffView");
+    const key = untitled.uri.toString();
+    const baseline = api.diffViewController.getBaseline(key);
+    assert.ok(baseline, "baseline captured for the untitled buffer with no folder");
+    assert.strictEqual(baseline!.reason, "opened");
+    // pin resets the baseline to now — works folder-less.
+    await vscode.commands.executeCommand("cowriting.pinDiffBaseline");
     await new Promise((r) => setTimeout(r, 300));
-    const opened = vscode.window.tabGroups.all.some((g) =>
-      g.tabs.some(
-        (t) =>
-          t.input instanceof vscode.TabInputTextDiff &&
-          t.input.original.scheme === "cowriting-baseline" &&
-          t.input.modified.toString() === untitled.uri.toString(),
-      ),
-    );
-    assert.ok(opened, "a cowriting-baseline diff opened for the untitled buffer with no folder");
+    assert.strictEqual(api.diffViewController.getBaseline(key)!.reason, "pinned", "pin works with no folder");
   });
 });