Replace the event-level n_sec confusion matrix with two step-resolved
secondary-multiplicity comparisons:
- sec_count_per_step: overlay histogram of how many secondaries a single
step emits, rollout series vs reference.
- sec_count_per_step_by_species: heatmap of per-step multiplicity of one
species (zero row included) against species, drawn as one panel per
rollout plus a reference panel, raw counts on a log color scale.
Both are backed by a new sources.secondaries_by_step view, which tags each
secondary with its emitting step — (event_id, parent_id, birth position)
on the rollout side, the row index on the reference side — so neither plot
needs a join against the step frame. Steps that emitted nothing are
recovered by subtraction from the chunk's step count, keeping both specs
sum-mergeable across condor chunks.
The rollout multiplicity is derived from the actual secondary birth rows
rather than the n_sec_pred column, which records the predicted count
before the per-event max-tracks cap.
_render_heatmap gained reference-panel and log-color support;
marginal_distance_summary sets neither key and is unchanged.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
lars