feat: add eval-cost benchmark — Geant4 reference vs surrogate rollout timing

Closes the roadmap's long-standing "no eval-latency number exists for any
configuration" gap. Instruments `giant rollout` to record per-physical-step
wall-clock cost in its YAML sidecar, adds a measured Geant4/miniCaloSim
per-step reference (giant/analysis/geant4_reference.py, from a 3-energy,
4-event-count-per-energy local benchmark), and wires both into a new
eval_cost_per_step PlotSpec in the giant analyze gallery.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_render.py

@@ -292,6 +292,20 @@ def test_render_one_of_each_kind(tmp_path: Path):
                 "ylabel": "frac",
             },
         ),
+        Reduced(
+            "cost",
+            "cost",
+            "bar",
+            "Cost",
+            "phase",
+            {
+                "labels": ["sample", "write", "total"],
+                "series": {"flow": [10.0, 1.0, 11.0]},
+                "reference": [5.0, 0.5, 5.5],
+                "ylabel": "us/step",
+                "log_y": True,
+            },
+        ),
         Reduced(
             "s",
             "species",