[beekld]

Summary

Behavior change: compared with the released 7.21.2, FDv2 status listeners that are still reading now receive an Off status when the client closes. Their channels then close as before. 7.21.2 neither broadcast Off nor set it on shutdown. #489 added a blocking broadcast that could hang Close(), and #498 removed it.

This PR adds Broadcaster.CloseWith, which sends a final value to each listener that has room for it in its buffer and then closes every listener channel. It never waits for a listener to read, so a listener that has stopped reading still can't block Close(). That listener misses Off and only sees its channel close.

The data system uses it in Stop() to send the Off status. As a result, a WaitFor(DataSourceStateOff, ...) call that's running when the client closes now returns true.

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Note

Overview
Adds Broadcaster.CloseWith, which non-blockingly pushes one final value into each subscriber channel that still has buffer space, then closes all listener channels. Close() is unchanged in behavior and routes through shared closeAll(nil) logic; RemoveListener still closes without a final value.

FDv2 Stop() now records DataSourceStateOff via maybeUpdateStatus and shuts down broadcasters with Close(status), so data-source status listeners that are still reading typically receive Off before their channel closes. WaitFor(DataSourceStateOff, …) can therefore return true when the client stops instead of timing out or failing only on channel close.

Listeners whose buffers are full still do not block shutdown—the final Off is dropped for them (same non-waiting guarantee as CloseWith). Tests cover broadcaster CloseWith semantics and FDv2 stop / WaitFor(Off) behavior.

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