photocraft_io::named_for_file(doc, name) serialises the document under the
file it is being written to, so a .pcraft manifest carries the identity the
save is about to adopt rather than the previous one — the same rule every
other format follows on import, where document.name comes from the file it
was read from. A bare extension names no file and changes nothing.
Applied once, in the .pcraft branch of export_stages, which export and
export_with both funnel through: the desktop Save (services.rs:242),
Export (services.rs:384), the web build (web.rs:355) and the engine file
command (file_cmds.rs:295) are covered by one site, and the previews
rendered before the bytes describe the same document as the manifest.
Scope per review: the automation funnels (files::save / save_bytes) already
landed in #3319 with their own named_for_output, so this carries no
crates/automation change; its scenarios are covered by
crates/automation/tests/native_save_name.rs on main.