docs(connection): describe native export handoff accurately

The fixture comment still said the Trajectory action used window.fetch after the implementation moved to a temporary download anchor. That wording implied client-side response handling and buffering which the browser-download design deliberately avoids.\n\nDescribe the actual native download-manager handoff while retaining the important contract: the fixture download stub only satisfies the host type and is unreachable through fixture dispatch.

packages/client/connection/src/client/fixture.ts

@@ -2835,8 +2835,8 @@ function createFixtureWorld(options: FixtureOptions): FixtureWorld {
       return Promise.resolve({ accepted: true })
     },
     // Satisfies the ApiProxy contract type only: the browser export button
-    // fetches GET /api/session.export directly (window.fetch), so this stub is
-    // never reached through the fixture's dispatch.
+    // hands GET /api/session.export to the native download manager, so this
+    // stub is never reached through the fixture's dispatch.
     downloads: {
       sessionLog: () => Promise.resolve(new Response('fixture mode does not serve session export', { status: 404 })),
     },