13 tests pin slskd HTTP API contract: endpoint format
(`transfers/downloads/<username>` POST), payload shape
(slskd web-interface array format), id extraction from dict /
list / fallback responses, and the username-lookup fallback in
cancel_download when no username hint is provided.
Phase A of the download engine refactor — pinning current
behavior of every source BEFORE moving any code so the engine
extraction can't drift the per-source contract. Includes the
plan doc at docs/download-engine-refactor-plan.md.
Pure additive — no client code changes.
