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perf(realtime): route inbound frames through a per-topic channel index (#1723)

* perf(realtime): route inbound frames through a per-topic channel index

The core found the channel of each frame by scanning every channel record,
which cost about 100 ns per record: about 100 us per frame at 1,000 channels,
more than decoding the frame. A join reply scanned twice.

RealtimeCore now keeps channelsByWireTopic, the channel ids of each wire topic
in creation order. setRecord(_:for:) keeps it in step when a record is added or
dropped. Routing looks only at the records of the frame's topic, and keeps its
rules: join_ref picks the record of that join or leave, and a frame without one
goes to the live channel that is not being removed.

* refactor(realtime): keep the channel records and the topic index in one ChannelTable

The records and their topic index were two properties of RealtimeCore, and any
write to the records could skip the index. One test did: it stored a record with
a subscript write, so routing could not find it.

ChannelTable now owns both, and they are private(set). setRecord(_:for:) is the
only way to add, replace, or drop a record. update(_:_:) changes a record in
place, so its storage is not copied, but it cannot add or drop a record, and a
record's wire topic never changes. So the compiler keeps the two in step.

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