Build on #114129 (@KoNit-K), which carries a Desktop-generated large-paste
preview from the composer through `prompt.submit` -> `display_metadata` ->
turn context -> the shared title input. Two gaps closed:
- `apply_instant_title` never received the preview, so the instant title of a
paste-only opener was the generated `@file:` path — and stayed that way,
because the upgrade thread's `derive_title` fallback writes `derived`
provenance, which never replaces the `derived` title already stored.
Thread the hint into the instant stage too.
- `build_title_input` let the `@file:` ref lead when the opener was nothing
but the generated attachment ref; the preview now leads for a ref-only
opener (an instruction still leads when the user typed one).
- `prompt.submit` gains `title_preview` in the contract (regenerated shared
TS/OpenRPC); documented as title-only input in the configuration guide.
- Tests trimmed to two invariants (shared input reaches both stages; budget +
manual attachments stay unread).
34ba61bf67