Make reading held screenshots confidential (BrowserReadOutput)

[senamakel]

Problem

A browser task can fill forms with a caller's facts (names, contact details, and under payment: fill_then_approve, card details) and then stops with a screenshot of that screen in TaskReport.artifacts (#60). TaskReport is confidential, so the screenshot's handle only reaches callers through a confidential frame. But the bytes are read with BrowserReadOutput, which is not a confidential member, so the image crosses the bus in ordinary frames.

The same is true of any BrowserScreenshot a caller takes of a page showing typed secrets.

Raised in review: #60 (comment)

Options

1. Mark BrowserReadOutput #[tinybus(confidential)]. It's the simplest and safest shape. But every current plain call starts failing, so under the contract rules it's a major bump (3.0). The in-memory test bus can't attest a module, so tests would call the service directly.
2. Add a confidential ReadTaskArtifact member (minor bump), and read task artifacts only through it. Nothing breaks, but it adds a second way to read an output, and BrowserScreenshot outputs stay unprotected.

Mitigations already in place

• Output ids are unguessable and expire after five minutes.
• Since Address review feedback from #52 and #57 #60, task screenshots appear only in the confidential TaskReport, never in a TaskView, AwaitTask or ListTasks reply.

[tinysweeper]

BrowserReadOutput is not marked confidential, allowing sensitive data (including payment card details) to cross the bus in plain frames. Task screenshots are already in a confidential TaskReport, but the byte-reading member is not. This is a broken core security path with no existing workaround.

Labelled priority: p1.