Runs one picked instance to a committed outcome.

Resolves the FSM module, loads the jsonb state into the FSM's struct, and calls step/2 under try. The signal-inbox and children snapshots ride in the job itself — batch-loaded by the pick, not fetched per step. A raised exception routes to handle/2, and so does an uncaught throw (as {:throw, value}); if handle/2 itself raises, the instance fails. A worker crash (a bare exit, a kill — no return at all) is not handled here — it is the reaper's job, the at-least-once safety floor.

Signal consumption: a step sees the awaited subset as ctx.awaited (only the signals whose name is in the set it parked on) and the whole inbox as ctx.all. On a progressing outcome the engine deletes exactly the ctx.awaited ids the step received — latecomers and never-awaited signals survive; a terminal outcome clears the whole inbox (cleanup); :retry/:await delete nothing. Deletion happens in SQL, by id.

Outcomes commit only while this worker still owns the claim (locked_by + status = 'executing' guard in every complete_*): an orphaned task whose lease expired and whose row was reclaimed gets its late outcome dropped — observable as [:gen_durable, :outcome, :stale] — and the current claimant redoes the step (at-least-once).