`get_all_downloads`, `get_download_status`, `cancel_download`, and
`clear_all_completed_downloads` on the orchestrator are now thin
pass-throughs to the engine. The plugin-iteration logic lives in
one place (the engine) instead of duplicated across orchestrator
methods.
Source-hint routing semantics preserved verbatim — engine.cancel
treats streaming-source names as direct routes and unknown names
as Soulseek peer usernames, exactly like the legacy orchestrator
did. Per-plugin exceptions still get swallowed defensively.
Test fixture `_build_orchestrator` now constructs an engine and
registers every mock plugin so the helper-built orchestrators
have the same wiring as production.
Suite still green (2012 passed). Zero behavior change for users.
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