feat(analysis): per-step secondary multiplicity plots

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>

tests/test_render.py

@@ -304,13 +304,15 @@ def test_render_one_of_each_kind(tmp_path: Path):
             "hm1",
             "secondaries",
             "heatmap",
-            "Confusion (single rollout)",
+            "Heatmap (single rollout)",
             "predicted",
             {
                 "series": {"flow": [[1, 0], [0, 1]]},
+                "reference": [[2, 0], [0, 1]],
                 "row_labels": ["0", "1+"],
                 "col_labels": ["0", "1+"],
                 "cbar_label": "count",
+                "log_color": True,
             },
         ),
     ]