`MediaServerEngine` reads the active server from config + dispatches
to the corresponding registered client. Per-server reaches still
work through `engine.client(name)`.
Required-method dispatch (is_connected, ensure_connection,
get_all_artists, get_all_album_ids) returns safe defaults when
the active client failed to initialize OR when the method raises.
Optional-method dispatch (search_tracks, trigger_library_scan,
is_library_scanning, get_library_stats, get_recently_added_albums)
checks hasattr first — SoulSync standalone has no
trigger_library_scan or get_library_stats, engine no-ops with
appropriate defaults instead of forcing every client to declare
stub methods.
10 new engine tests pin: active-server resolution, required
dispatch routing, exception safety, missing-optional-method
fallback shape. Suite still green (1951 passed).
Engine isn't on any production code path yet — Phase C migrates
the 33 web_server.py dispatch sites to call engine.method()
instead of hand-branching by active_server name.
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