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>

src/trackChangesPreview.ts

@@ -18,6 +18,17 @@ import { renderReview, renderPlain, diffBlocks, type BlockOp } from "./trackChan
 const VIEW_TYPE = "cowriting.trackChangesPreview";
 const DEBOUNCE_MS = 150;
 
+/**
+ * Inbound webview→host messages (intent only — the sealed webview never mutates,
+ * INV-21/35). F10 carried the annotations toggle + ✓/✗ proposal decisions; F11
+ * adds the toolbar intents (pin baseline / ask Claude).
+ */
+type ToolbarMsg =
+  | { type: "setMode"; mode: "on" | "off" }
+  | { type: "accept"; proposalId: string }
+  | { type: "reject"; proposalId: string }
+  | { type: "pinBaseline" };
+
 export class TrackChangesPreviewController implements vscode.Disposable {
   private readonly disposables: vscode.Disposable[] = [];
   private readonly panels = new Map<string, vscode.WebviewPanel>();
@@ -91,21 +102,10 @@ export class TrackChangesPreviewController implements vscode.Disposable {
       null,
       this.disposables,
     );
-    // F10: the webview posts the annotations toggle + ✓/✗ proposal decisions back.
+    // F10/F11: the webview posts the annotations toggle, ✓/✗ proposal decisions,
+    // and (F11) the toolbar intents (pin baseline / ask Claude) back to the host.
     panel.webview.onDidReceiveMessage(
-      (m: { type?: string; mode?: "on" | "off"; proposalId?: string }) => {
-        if (m?.type === "setMode" && (m.mode === "on" || m.mode === "off")) {
-          this.mode.set(key, m.mode);
-          this.refresh(document);
-        } else if (m?.type === "accept" && m.proposalId) {
-          void this.proposals
-            .acceptById(this.proposals.keyFor(document), m.proposalId)
-            .then(() => this.refresh(document));
-        } else if (m?.type === "reject" && m.proposalId) {
-          this.proposals.rejectById(this.proposals.keyFor(document), m.proposalId);
-          this.refresh(document);
-        }
-      },
+      (m: ToolbarMsg) => this.handleWebviewMessage(document, m),
       null,
       this.disposables,
     );
@@ -115,6 +115,30 @@ export class TrackChangesPreviewController implements vscode.Disposable {
     this.refresh(document);
   }
 
+  /**
+   * Route an inbound webview intent through the existing seams (INV-35): the
+   * annotations toggle + ✓/✗ proposal decisions (F10) and the toolbar gestures
+   * (F11). Pin targets the PREVIEWED document (`DiffViewController.pin`), not the
+   * active editor — the preview knows its bound doc (§6.7).
+   */
+  private handleWebviewMessage(document: vscode.TextDocument, m: ToolbarMsg): void {
+    const key = document.uri.toString();
+    if (m?.type === "setMode" && (m.mode === "on" || m.mode === "off")) {
+      this.mode.set(key, m.mode);
+      this.refresh(document);
+    } else if (m?.type === "accept" && m.proposalId) {
+      void this.proposals
+        .acceptById(this.proposals.keyFor(document), m.proposalId)
+        .then(() => this.refresh(document));
+    } else if (m?.type === "reject" && m.proposalId) {
+      this.proposals.rejectById(this.proposals.keyFor(document), m.proposalId);
+      this.refresh(document);
+    } else if (m?.type === "pinBaseline") {
+      // F6 baseline store re-render arrives via the onDidChangeBaseline subscription.
+      this.diffView.pin(document);
+    }
+  }
+
   private onEdit(document: vscode.TextDocument): void {
     const key = document.uri.toString();
     if (!this.panels.has(key)) return;
@@ -232,6 +256,7 @@ export class TrackChangesPreviewController implements vscode.Disposable {
 <body>
   <div id="cw-header">
     <label id="cw-toggle"><input type="checkbox" id="cw-annotations" checked /> Annotations</label>
+    <button id="cw-pin" type="button" title="Pin the review baseline to now (clears the change-marks)">⌖ Pin baseline</button>
     <span id="cw-epoch">Review</span>
     <span id="cw-summary"></span>
     <span id="cw-legend"></span>
@@ -246,6 +271,15 @@ export class TrackChangesPreviewController implements vscode.Disposable {
   isOpen(uriString: string): boolean {
     return this.panels.has(uriString);
   }
+  /**
+   * F11 test seam: deliver an inbound webview message to the real routing, as if
+   * the sealed webview had posted it. Exercises message→seam wiring without a
+   * live webview DOM (which is manual-smoke only). No-op if no doc/panel.
+   */
+  receiveMessage(uriString: string, m: ToolbarMsg): void {
+    const doc = vscode.workspace.textDocuments.find((d) => d.uri.toString() === uriString);
+    if (doc && this.panels.has(uriString)) this.handleWebviewMessage(doc, m);
+  }
   getLastModel(uriString: string): BlockOp[] | undefined {
     return this.lastModel.get(uriString);
   }