Support client events (client-*)

[stephenjason89]

Goal

Allow clients to trigger client-* prefixed events on private and presence channels (pusher-js channel.trigger('client-foo', data)), per the Pusher protocol. Currently these arrive at handleWebSocketMessage and hit the "Unhandled Event" branch.

Requirements (per Pusher spec)

• Only allow on private- / presence- channels the sender is actually subscribed to; reject elsewhere with a pusher:error
• Broadcast to all other subscribers, excluding the sender (ws.publish does this natively in Bun)
• Enforce the event-name prefix client- and a payload size limit (Pusher: 10KB)
• Optionally emit the client_event webhook once webhook support (Webhook Support #3) expands

Depends on per-channel subscription state (#12) to verify the sender's membership.

Promoted from the README "Contributing > Client-Side Events" section so it can be tracked.