Per JohnBaumb: the single state_lock serialized progress callbacks
across every source. Pre-refactor each client owned its own download
lock, so Deezer / YouTube / Tidal workers never blocked each other.
Multi-source concurrent downloads under the unified lock fought for
the same RLock on every progress update.
Replaced the engine-wide state_lock with per-source RLocks. Each
source gets its own lock, lazily created via _source_lock() on first
use (meta-lock guards the create-race). All record mutations
(add/update/update_unless_state/remove/get/iter) take only that
source's lock — Deezer progress updates no longer block Tidal writes.
Cancelled-preserve semantics still hold because cancel + worker
terminal write target the same source, so they share that source's
lock. New test pins lock independence: holding source-A's lock from
one thread does not block a write on source-B from another.
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