system traces: producer array shared by two clients

systemTraceProducers holds producer instances. If the same array is passed to two clients, both containers start/stop the same producers and the idempotent start/stop plus held-value state interleave. Currently documented only.

Options: accept a factory instead of instances, or detect and reject double ownership. Decide and pin with a test.

Deferred from PR #55 (trace producers over shared trackers).