Strict

Multi-threaded with whatever synchronisation each stat needs to stay exact across coupled state. For Welford-style stats this is a lock around the body of update. For sketches that already self-serialise (DDSketch, t-digest, HDR, and so on) Strict is the natural mode and adds no extra overhead.

Every writer goes through the same lock for coupled stats. Shard the work and merge when the write rate swamps a single lock.