A venue's price feed: symbols in, price events out. How is the venue's business.

The boundary this restores

The application asks a venue for a feed over a symbol set and receives price events. It does not ask how they are produced. A venue may use a websocket, five MQTT sessions, an internal REST poll, or any mix of those, and may change that mix at runtime without anything outside the venue noticing.

This is the design the codebase already claims — hexagonal, with the exchange as the adapter — and the collection layer had drifted a long way from it. Orchestration accreted in a shared collection layer one venue at a time, until it held Webull's session count, Gemini's ten-pairs-per-socket limit, Coinbase's channel ordering, and a case provider do for which module speaks which protocol. Every one of those is knowledge only the venue has.

The cost of that drift was not tidiness. Shared code was making transport decisions with information it could not have:

• PollSet had to guess which pairs a subscription was covering, because the only honest answer lives inside the venue.
• Webull's documented ceiling of "3 messages per second per connection" was being rationed by a module that could not see it, so 151 subscribed pairs delivered nothing and it read as a quiet market.
• A venue with no socket at all was reported to the user in socket terms, because the layer describing it was inferring transport rather than being told.

Always a feed, even when it is really a poll

A venue with no streaming API still implements this. Robinhood polls REST internally and emits events through the same sink as Coinbase's socket, so nothing above the adapter branches on transport. That is the point: the poll and the socket become an implementation detail of the venue rather than a fork that every consumer has to understand.

Coverage is OBSERVED, never intended

coverage/1 must report what the feed has actually seen arrive, not what it subscribed. The distinction is the whole value of the callback. Webull subscribed 325 symbols and confirmed them while 174 delivered; a feed reporting its subscription would have answered "325 covered by socket" — the same wrong answer the collection layer was already inferring, but stamped with the venue's authority, which is worse than an inference known to be a guess.

A feed that cannot observe its own delivery reports :not_covered rather than assuming. Silence is the honest answer to a question it cannot answer.

This does not replace independent measurement

TickFreshness stays, and stays independent. This callback is what the venue BELIEVES it is covering; the freshness table is what actually arrived at the publisher. Keeping both means their disagreement is visible, and that disagreement is the highest-value signal in the system — "the venue claims 325 and 44 arrived" states an entire morning's investigation in one line. Folding them into a single number would throw exactly that away.