stage_key formatted "<execution>:<firing>" and five places re-derived or
re-parsed that string: the engine, progress and platform folds, the
projector's ordering rule, and fork.rs's stage_labels, which split it
back apart. FiringKey is the fact itself, with of_event for the event
side and From<StagePosition> for the platform-record side. It still
serializes as "<execution>:<firing>", so the fold_json a stored view
holds keeps its shape and needs no migration; a unit test pins that.
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>