Log broadcasts once per event instead of once per subscriber (#73)

[russ]

Problem

Fixes #73.

Cable::Server#send_to_channels logged the "transmitting" line inside the per-subscriber loop:

@channels[channel_identifier].each do |channel|
  ...
  Cable::Logger.info { "#{channel.class} transmitting #{parsed_message} (via streamed from #{channel.stream_identifier})" }
  channel.connection.socket.send(...)
end

So broadcasting one message to a stream with N connected clients produced N identical log lines. A chat room with 100 viewers logged "hi" 100 times; the only workaround was Cable::Logger.level = :none, which silences all Cable logging.

Change

Move the log outside the loop and emit a single line per broadcast event, including the number of subscribers it was delivered to:

Transmitting {"message" => "hi"} to 13 subscribers (via streamed from chat_room_1)

• Counts only live connections actually transmitted to (closed connections are still reaped in the loop as before).
• Skips the log entirely when there are no live subscribers.

The redundant per-message broadcast is still logged once at the call site (Channel#broadcast / .broadcast_to), so no information is lost.

Tests

Added spec/cable/server_spec.cr#send_to_channels — subscribes two connections to the same stream, captures logs, and asserts exactly one info line (... to 2 subscribers ...) is emitted. Full suite green (52 examples, 0 failures); ameba clean.

Note

This changes the log line format (<Class> transmitting <msg> (via streamed from <id>) → Transmitting <msg> to <n> subscribers (via streamed from <id>)). Anything grepping the old string will need updating.

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