AmazonDownloadClient was missing set_engine() and set_shutdown_check().
The download engine auto-wires plugins by calling set_engine(self) at
registration time if the method exists (engine.py:136). Without it,
_engine stayed None forever, causing every download() call to raise
RuntimeError("_engine is not set") — silently failing and marking all
tracks not found.
All other streaming clients (Deezer, Qobuz, Tidal, HiFi, SoundCloud)
expose set_engine(); Amazon now matches the pattern.
Tests added: set_engine wires _engine, set_shutdown_check wires callback,
set_engine unblocks download dispatch (the exact live failure mode).
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