Function try_fetch_paper_plan 

pub fn try_fetch_paper_plan(
    ref_: &Ref,
    store_root: &Utf8Path,
) -> Result<FetchPlan, FetchError>

Fallible sibling of fetch_paper_plan — propagates an internal allowlist-contract drift as a typed FetchError::SourceSchema instead of degrading to an empty candidate_hosts list (issue #156 ②). Thin re-export of crate::dry_run::try_build_fetch_plan. Added alongside the infallible fetch_paper_plan rather than changing its signature, because fetch_paper_plan is pub and called from doiget-mcp, which is out of scope for this batch.

Errors

See crate::dry_run::try_build_fetch_plan.