feat(f6): wire DiffViewController into activation (SLICE-3)

F6 §6.2. BaselineStore from context.storageUri (in-memory fallback), advance
subscribed to the seam's onDidApplyAgentEdit, ensureBaseline on first sight in
renderIfOpen, diffViewController on the CowritingApi, commands stubbed in the
no-folder path (#8 precedent).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/extension.ts

@@ -6,6 +6,8 @@ import { AttributionController } from "./attributionController";
 import { ProposalController } from "./proposalController";
 import { buildFingerprint } from "./anchorer";
 import { VersionGuard } from "./versionGuard";
+import { BaselineStore } from "./baselineStore";
+import { DiffViewController } from "./diffViewController";
 
 const CHANNEL_NAME = "Cowriting (Cline SDK)";
 
@@ -14,6 +16,7 @@ export interface CowritingApi {
   attributionController: AttributionController;
   proposalController: ProposalController;
   versionGuard: VersionGuard;
+  diffViewController: DiffViewController;
 }
 
 export function activate(context: vscode.ExtensionContext): CowritingApi | undefined {
@@ -63,6 +66,8 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
       "cowriting.acceptProposal",
       "cowriting.rejectProposal",
       "cowriting.proposeAgentEdit",
+      "cowriting.toggleDiffView",
+      "cowriting.pinDiffBaseline",
     ]) {
       context.subscriptions.push(vscode.commands.registerCommand(command, stub));
     }
@@ -83,6 +88,18 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
   const proposalController = new ProposalController(store, attributionController, root, versionGuard);
   context.subscriptions.push(proposalController);
 
+  // --- F6: diff-view toggle (Feature #17) ---
+  // Baseline lives in VS Code workspace storage, never the repo (INV-19).
+  // storageUri can be undefined in odd host states → in-memory fallback (§6.5).
+  const storageDir = context.storageUri?.fsPath;
+  const baselineStore = storageDir ? new BaselineStore(storageDir) : null;
+  const diffViewController = new DiffViewController(baselineStore, root);
+  context.subscriptions.push(diffViewController);
+  // The seam's single machine-landing signal advances the baseline (INV-18).
+  context.subscriptions.push(
+    attributionController.onDidApplyAgentEdit((e) => diffViewController.advance(e.document)),
+  );
+
   // One SHARED sidecar watcher for both controllers; self-writes are
   // suppressed centrally in the store (the sidecar is co-owned).
   const watcher = vscode.workspace.createFileSystemWatcher("**/.threads/**/*.json");
@@ -228,12 +245,13 @@ export function activate(context: vscode.ExtensionContext): CowritingApi | undef
       threadController.renderAll(doc);
       attributionController.loadAll(doc);
       proposalController.renderAll(doc);
+      diffViewController.ensureBaseline(doc);
     }
   };
   vscode.workspace.textDocuments.forEach(renderIfOpen);
   context.subscriptions.push(vscode.workspace.onDidOpenTextDocument(renderIfOpen));
 
-  return { threadController, attributionController, proposalController, versionGuard };
+  return { threadController, attributionController, proposalController, versionGuard, diffViewController };
 }
 
 export function deactivate(): void {