Stop broadcaster subscribe/unsubscribe from taking the stop signal

[jonyoder]

Subscribe, SubscribeOne and Unsubscribe all selected on stopSignal, so a call in flight when the single stop value was sent could take it. broadcast() then never stopped or closed stopSignal, and whoever sent the stop waited forever. Callers now wait on an internal done channel that broadcast() closes when it exits, so only broadcast() reads stopSignal.

This fixes the hang behind rstudio/package-manager#21066, and PPM can drop the wrapper added in rstudio/package-manager#21068.

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Testing: New TestStopDuringSubscribe runs 50 goroutines calling Subscribe/SubscribeOne/Unsubscribe in a loop, sends a stop, and expects stopSignal to close within 1s. It fails on the first trial without the fix and passes -race -count=20 with it. just lint and just vet are clean.

Note: pkg/rsqueue/impls/database/queue.go (DatabaseQueue.SubscribeOne/Unsubscribe vs. its broadcast()) has the same pattern on stopChan. It isn't fixed here because WithDbTx copies don't carry stopChan, so it needs its own change.

🤖 Generated with Claude Code

[jonyoder]

Approved by @jonyoder in review of rstudio/package-manager#21068. Admin-merging on his behalf. Follow-up for the same race in DatabaseQueue: #314.