moq-lite: port Origin API renames from #1358

[kixelated]

Stacked on #1378.

Ports the public-API renames from #1358 without the new state model. The existing OriginNode tree, web_async::spawn cleanup, and tokio mpsc fan-out stay untouched.

Renames

• OriginProducer::publish_only -> scope
• OriginConsumer::consume_only -> scope
• OriginConsumer::try_consume_broadcast -> get_broadcast
• OriginProducer::consume_only and try_consume_broadcast: dropped (write producer.consume().scope(p) / .get_broadcast(p) instead)

Rationale: every consume_xxx on OriginProducer was a one-liner shortcut for producer.consume().xxx(). Dropping them keeps the producer surface focused on publishing. The try_ and consume_ prefixes were redundant on OriginConsumer.

New BroadcastConsumer primitives

• is_closed() and poll_closed(waiter) — thin wrappers over conducer::Consumer. Lets callers compose close-detection without spawning a task per broadcast.

NOT in this PR

• The new HashMap+Mutex state model.
• OriginProducer clone-as-refcount + Drop-closes-on-last (behavior change tied to the new model).
• OriginConsumer::is_closed / poll_next (channel close signal — net-new, not a rename).
• OriginUpdate enum — keeping the existing OriginAnnounce tuple.
• Suffix churn (publish_broadcast -> publish, etc.) — separate crate-wide question, deferred.

Test plan

• cargo check --workspace
• cargo test -p moq-lite --lib model::origin (41/41 origin tests pass)
• cargo test --workspace --lib
• just check
• Spot-check relay + clock end-to-end

🤖 Generated with Claude Code