v2: File upload and download handling

[loyality7]

Context

v1 has no way to drive a <input type=file> or catch a download. Both are common automation needs (form submission with an attachment, exporting a report).

Scope

• Upload: set files on a file input without a native file picker UI appearing.
• Download: intercept a download, cap its size, let the caller choose the destination.

Acceptance criteria

• page.setInputFiles(selector, paths) uploads without any picker UI
• A download is captured, size-capped, and written to a caller-chosen destination
• Oversized downloads are refused, not silently truncated
• Device tests cover both against fixture-site forms