Artifacts: add a native download button for the raw published file

Problem

When an Artifact is a generated file meant to be shared or forwarded as-is (e.g. a standalone HTML report), there's currently no way for a viewer to get the raw, unwrapped bytes of the published file from the artifact page itself.

• The published page is served through a wrapping viewer (theme/frame-runtime injection), not the original file verbatim.
• Per the Artifact tool's own documented sandbox constraints, a page cannot offer its own download affordance — <a download> links (including data:/blob: hrefs) and script-driven saves are inert for viewers. This is a deliberate sandbox restriction, not a bug in any specific artifact.
• Today, the only way to get the raw file out is for whoever has the authoring session to manually re-send it via a separate mechanism (e.g. a CLI's own file-attach feature) — there is no path for someone who was just sent the artifact link to get the raw file themselves.

Request

Add a native "Download" control to the Artifact viewer chrome itself (outside the sandboxed page content), so anyone with view access to an artifact can download the original published file directly — without needing the authoring session to re-share it through a side channel.

Example use case

Generated an ~8MB standalone HTML report via a CLI tool and published it as an Artifact so it has a stable, shareable permalink. A recipient wanted to forward the raw HTML file itself (e.g. to email it, or open it outside claude.ai) rather than the live claude.ai-wrapped page. There was no way to do that from the artifact page/link alone.