`DownloadEngine` grows async query methods that wrap plugin
iteration: `get_all_downloads` (concatenates every plugin's
active downloads), `get_download_status` (first plugin to
recognize the id wins), `cancel_download` (with source-hint
routing — streaming sources go direct, unknown hints route to
Soulseek as peer username), and `clear_all_completed_downloads`
(skips unconfigured plugins).
Code moved from the orchestrator's hand-iterated loops into the
engine. Orchestrator delegation comes in B3 — for B2 the engine
methods exist but nothing calls them yet.
Per-plugin behavior preserved verbatim (defensive `try ... except`
swallows per-iteration, unconfigured-skip on clear, source-hint
routing semantics). Phase A pinning tests + 8 new engine query
tests catch any drift.
Pure additive — zero behavior change for users.
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