ossia: do not queue control outputs nobody reads

[jcelerier]

finish_run enqueues the control outputs onto outs_queue on every tick where any of them was written. The host does not always install a reader — ossia score only does so when its execution update setting is on — and an unread moodycamel queue grows without bound, allocating a block inside the audio callback every few hundred ticks.

A node that writes a control output every tick, which is the natural shape for a playhead or a meter, therefore leaks for the length of the run.

controls_output_queue<T> gains:

std::atomic_bool notify_ui{true};

checked before the enqueue. The outputs_set bitset is still set and reset as before — it costs nothing and other code reads it; it is the enqueue that allocates.

Defaults to true, so every binding that does not set it (Max, PD, VST, standalone) behaves exactly as today. Only a host that explicitly opts out is affected.

Read on the audio thread, written by the host at setup; relaxed ordering is sufficient, since a stale read costs at most one extra or one skipped enqueue and nothing else depends on the ordering.

Needed by ossia/score#2295, which sets it from the setting that decides whether the reader is installed.

🤖 Generated with Claude Code

https://claude.ai/code/session_01H4N6KsxzvT1o9qoPvBThyz