The three It blocks that build a tight-cap streamer each spawned a fresh
transcoding cache without waiting for its background initialization. The
init goroutine reads conf.Server.CacheFolder, which races against
SnapshotConfig's pointer-swap restore (Server = &restored) fired by
DeferCleanup at the end of the spec. CI tripped the race under
-shuffle=on -race; locally it reproduced about 10% of the time.
Wait for tightCache.Available() before constructing the streamer, mirroring
the outer BeforeEach. For the slot-saturation spec, swap in a blocking
io.Pipe-backed mock ffmpeg so the cache's background copyAndClose can't
drain the source and release the slot — the previous behavior happened to
work only because the cache wasn't yet available and the no-cache path was
exercised.
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