Function fetch_paper 

pub async fn fetch_paper(
    ref_: &Ref,
    profile: &CapabilityProfile,
    ctx: &FetchContext,
    store: &dyn Store,
    store_root: &Utf8Path,
) -> Result<FetchPaperOutcome, FetchError>

Resolve a Ref to a PDF (or metadata-only fallback) and write it through store.

Binding spec: docs/MCP_TOOLS.md §4 (doiget_fetch_paper), docs/REDIRECT_ALLOWLIST.md §3 (informed-best-effort posture for the DOI OA PDF leg), docs/PROVENANCE_LOG.md §3 (per-attempt Fetch rows emitted by the source impls; StoreWrite row emitted by this orchestrator).

Dispatch

• Ref::Arxiv(_) → ArxivSource::fetch; the source returns PDF bytes + Atom-feed metadata. The orchestrator writes both the PDF and the metadata TOML.
• Ref::Doi(_) → Crossref metadata + Unpaywall license/OA-URL enrichment + (when the OA URL host is on the oa-publisher allowlist) a publisher PDF leg. A failure on the PDF leg is non-fatal: the metadata is still written and the orchestrator returns Ok(...) with source set to the metadata source.

Side effects

Each consulted source emits one LogEvent::Fetch row via ctx.log.append. The orchestrator additionally emits one LogEvent::StoreWrite row on the successful write. Session bookend rows are the caller’s responsibility (the CLI’s commands::fetch::run_with_options wraps the call; the MCP server’s doiget_fetch_paper tool method wraps it too).

Errors

Returns FetchError from the underlying Source dispatch. The MCP boundary converts these to the closed crate::ErrorCode set via the existing From<FetchError> for ErrorCode impl.