feat(f11): SLICE-1 — Pin baseline toolbar button + reachability (#43)

Homes the orphaned cowriting.pinDiffBaseline command and gives the writer a
reachable Pin control in the preview toolbar. Per spec §7.2 SLICE-1
(docs/superpowers/specs/2026-06-12-f11-preview-toolbar-interaction-surface.md).

- trackChangesPreview: extract onDidReceiveMessage into handleWebviewMessage;
  add the F11 `pinBaseline` intent → DiffViewController.pin(previewedDoc) (the
  bound doc, not activeTextEditor — §6.7); ToolbarMsg union; receiveMessage test
  seam exercising the real message→seam wiring (INV-35).
- webview: ⌖ Pin baseline button in #cw-header posting { type: "pinBaseline" };
  theme-aware toolbar-button CSS (light/dark/high-contrast, disabled state).
- package.json: unhide pinDiffBaseline — commandPalette `when` false →
  editorLangId == markdown (resolves the #34 orphan from the command side).
- host E2E (test/e2e/suite/f11Toolbar.test.ts): pinBaseline message clears the
  change-marks + advances the baseline to `pinned`; palette `when` is reachable.

Also archives the F11 implementation plan to docs/superpowers/plans/.

197 unit + 47 host E2E green.

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

test/e2e/suite/f11Toolbar.test.ts

@@ -0,0 +1,69 @@
+import * as assert from "assert";
+import * as fs from "fs";
+import * as path from "path";
+import * as vscode from "vscode";
+import type { CowritingApi } from "../../../src/extension";
+
+const WS = process.env.E2E_WORKSPACE!;
+const settle = () => new Promise((r) => setTimeout(r, 400));
+
+async function getApi(): Promise<CowritingApi> {
+  const ext = vscode.extensions.getExtension("benstull.vscode-cowriting-plugin")!;
+  const api = (await ext.activate()) as CowritingApi;
+  assert.ok(api?.trackChangesPreviewController && api?.diffViewController, "exports preview + diffView");
+  return api;
+}
+
+/** Create + open a fresh markdown doc under WS, returning the doc + its uri key. */
+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 });
+  fs.writeFileSync(abs, body, "utf8");
+  const uri = vscode.Uri.file(abs);
+  const doc = await vscode.workspace.openTextDocument(uri);
+  await vscode.window.showTextDocument(doc);
+  await settle();
+  return { doc, key: uri.toString() };
+}
+
+// F11 host E2E (no LLM): the preview toolbar is the primary interaction surface.
+// The webview posts intent messages; the host routes them through the existing
+// F4/F6/F3 seams (INV-35). The webview DOM (real button clicks) is sealed and
+// manual-smoke only; here we simulate the inbound messages via `receiveMessage`.
+suite("F11 preview toolbar (host E2E — message → seam wiring, no LLM)", () => {
+  // SLICE-1: the Pin baseline button.
+  test("pinBaseline message pins the PREVIEWED doc → marks clear, baseline reason is pinned (PUC-5, INV-35)", async () => {
+    const { doc, key } = await freshDoc("docs/f11pin.md", "# F11 pin\n\nA baseline paragraph that will diverge.\n");
+    const api = await getApi();
+    await vscode.commands.executeCommand("cowriting.showTrackChangesPreview");
+    await settle();
+    assert.strictEqual(api.trackChangesPreviewController.isOpen(key), true, "panel open");
+
+    // Diverge from the opened baseline so the preview carries a real change-mark.
+    const edit = new vscode.WorkspaceEdit();
+    edit.insert(doc.uri, doc.positionAt(doc.getText().length), "\n\nA freshly typed paragraph that diverges.\n");
+    assert.ok(await vscode.workspace.applyEdit(edit), "operator edit applied");
+    await settle();
+    const marked = (api.trackChangesPreviewController.getLastModel(key) ?? []).some((o) => o.kind !== "unchanged");
+    assert.ok(marked, "the typed paragraph shows as a change before pinning");
+
+    // Simulate the webview's Pin baseline button posting its intent.
+    api.trackChangesPreviewController.receiveMessage(key, { type: "pinBaseline" });
+    await settle();
+
+    const model = api.trackChangesPreviewController.getLastModel(key) ?? [];
+    assert.ok(model.length > 0 && model.every((o) => o.kind === "unchanged"), "after pin, every block is unchanged");
+    assert.strictEqual(api.diffViewController.getBaseline(key)?.reason, "pinned", "baseline reason advanced to pinned");
+  });
+
+  // SLICE-1 reachability: the orphaned pin command gets a real palette `when`.
+  test("pinDiffBaseline is reachable from the command palette (when: editorLangId == markdown)", async () => {
+    const pkg = JSON.parse(fs.readFileSync(path.join(__dirname, "../../../../package.json"), "utf8"));
+    const entry = (pkg.contributes.menus.commandPalette as Array<{ command: string; when?: string }>).find(
+      (m) => m.command === "cowriting.pinDiffBaseline",
+    );
+    assert.ok(entry, "pinDiffBaseline has a commandPalette entry");
+    assert.notStrictEqual(entry!.when, "false", "it is no longer hidden (when:false)");
+    assert.match(entry!.when ?? "", /editorLangId == markdown/, "guarded on markdown");
+  });
+});