Decide default concurrency for registered operators #949

Summary

Decide what the default concurrency should be for each kind of registered operator. The registration API currently defaults fold, observe, predicate, provide, transform, unfold, and output to concurrency::serial (see phlex/core/graph_proxy.hpp). Users can override the concurrency explicitly when registering an operator.

This issue is for discussing whether a single default remains appropriate across operator kinds, or whether some operators should default to a different concurrency (for example, concurrency::unlimited or a framework-selected limit).

Discussion points

• What safety and usability assumptions should defaults make about user algorithms?
• Should defaults differ by operator kind?
• What compatibility and migration implications would changing an existing default have?
• What documentation should explain the chosen behavior?

Scope

Reach and document a decision for the default concurrency of each registered operator kind. This issue does not prescribe a particular default or require changing the API before the tradeoffs are discussed.