Adds `RemoveSource` to stop streaming from a previously registered source. Removing a source cancels its connection loop and closes any in-flight websocket so the loop exits promptly instead of waiting for the next reconnect tick. Per-source runtime state (the loop's cancel func and the active websocket conn) was previously split across two sync.Maps and a separate map of registered sources, each with its own locking. That left several races, e.g. the conn was stored in connMap only after a successful dial, so a remove during the dial would miss it. The new design collapses everything into a single `sources map[Source]*sourceState` guarded by `sourcesMu`. The lock is only held for short, non-blocking map mutations and is always released before any side effect possibly-blocking calls. `runConnection` now re-checks under the lock that the source is still registered (and the ctx not cancelled) before installing a freshly dialed conn, and its deferred cleanup only clears the conn slot if the entry still points at the conn it installed. This makes 'remove during dial' and 'remove during read' both deterministic: the cancel cuts the loop, the close breaks ReadMessage, and a concurrent runConnection cannot resurrect state for a removed source.