Reproduces Soul's probe directly: 20,000 client-sent latest-only frames
on one key, none ever drained by receive(), leave exactly 1 pending
frame in the provider's receive queue, and that frame carries the
newest sequence sent. A second test sends 20 frames across 50 distinct
keys and checks the queue holds exactly 50 (bounded by key count, not
frame count). A third proves reliable-ordered client frames are never
coalesced and stay in order at the provider.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
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