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_analysis_reduce.py

@@ -13,6 +13,7 @@ from giant.analysis.sources import (
     open_side,
     physical_steps,
     secondaries,
+    secondaries_by_step,
 )
 from giant.data.loader import EVENT_ID_FILE_STRIDE
 
@@ -159,18 +160,17 @@ def test_secondaries_rollout_vs_reference_align():
     assert t["pdg"].to_list() == [22, 22]
 
 
-def test_sec_count_by_event_zero_fills_events_with_no_secondaries():
-    r_phys = physical_steps(_rollout_frame(), Side.rollout)
-    r_sec = secondaries(_rollout_frame(), Side.rollout)
-    ev, n = R.sec_count_by_event(r_phys, r_sec)
-    # event 1 has one secondary track; event 2 has none and must still appear (as 0),
-    # not silently drop out of a plain group_by on the secondaries frame alone.
-    assert dict(zip(ev.tolist(), n.tolist())) == {1: 1, 2: 0}
+def test_secondaries_by_step_keys_each_secondary_to_its_emitting_step():
+    r = secondaries_by_step(_rollout_frame(), Side.rollout).collect()
+    assert r["pdg"].to_list() == [22]
+    # the rollout key is (event_id, parent_id, birth position) — the parent
+    # step's post_pos, copied verbatim onto the child's birth row.
+    assert r["step_key"][0] == {"event_id": 1, "parent_id": 0, "pre_x": 0.0, "pre_y": 0.0, "pre_z": 1.0}
 
-    t_all = _reference_frame()
-    t_sec = secondaries(t_all, Side.reference)
-    ev, n = R.sec_count_by_event(t_all, t_sec)
-    assert dict(zip(ev.tolist(), n.tolist())) == {1: 1, 2: 1}
+    t = secondaries_by_step(_reference_frame(), Side.reference).collect()
+    assert t["pdg"].to_list() == [22, 22]
+    # one row per emitting step; the empty-list step drops out entirely
+    assert [k["_row"] for k in t["step_key"]] == [0, 2]
 
 
 def test_leakage_fraction():