Outcome::Scored becomes a struct variant with an Option<f64> sigma
field. None inherits HistoryBuilder::score_sigma; Some(s) overrides
per event. Resolved at ingest time so EventKind::Scored stays a plain
f64 and TimeSlice/run_chain need zero changes. New constructors
Outcome::scores_with_sigma and EventBuilder::scores_with_sigma cover
the override path; existing scores(..) keeps its signature with
sigma=None internally.
Breaking change to Outcome::Scored variant shape (tuple → struct);
acceptable in 0.1.x. Closes the last item from the T4-MarginFactor
deferred wishlist.
