Update an experiment (with status-based field allowlist)

Use when: caller wants to change an experiment’s metadata, configuration, or status (start, pause, complete) — what fields are editable depends on current status

Partial update of an experiment. Allowed fields depend on the experiment’s current status: draft allows nearly all fields, running/paused allows only metadata + operational fields (rate sampling, judge eval rate, etc.), terminal states allow only metadata. Disallowed fields return 400 naming what was rejected. Status transitions are validated — only running / paused / stopped accepted; running a new split experiment on a gate that already has one returns 409 with conflict metadata. Stopping an experiment records its outcome via recordExperimentOutcome for the Architect’s reward-signal aggregation.