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_cli_rollout.py

@@ -8,11 +8,52 @@ from __future__ import annotations
 import torch
 from typer.testing import CliRunner
 
-from giant.cli import app
+from giant.cli import _build_rollout_timing, app
 
 runner = CliRunner()
 
 
+def test_build_rollout_timing_excludes_synthetic_rows_from_per_step_cost():
+    # 100 rows total, 30 of them synthetic termination markers (escape) ->
+    # us_per_step should be normalized over the 70 physical rows only, the
+    # same unit giant.analysis.geant4_reference measures Geant4 in.
+    timing = _build_rollout_timing(
+        setup_s=1.0,
+        rollout_s=10.0,
+        write_s=2.0,
+        n_rows=100,
+        termination_reason_counts={"escaped": 30, "natural_end": 70},
+        n_seed_events=5,
+        device="cpu",
+        torch_threads=4,
+    )
+    assert timing["n_rows"] == 100
+    assert timing["n_physical_rows"] == 70
+    assert timing["n_physical_rows"] < timing["n_rows"]
+    assert timing["sample_s"] == 8.0  # rollout_s - write_s
+    assert timing["us_per_step"] == 8.0 / 70 * 1e6
+    assert timing["write_us_per_step"] == 2.0 / 70 * 1e6
+    assert timing["ms_per_event"] == 10.0 / 5 * 1e3
+    assert timing["device"] == "cpu" and timing["torch_threads"] == 4
+
+
+def test_build_rollout_timing_handles_zero_physical_rows_and_events():
+    timing = _build_rollout_timing(
+        setup_s=1.0,
+        rollout_s=1.0,
+        write_s=0.0,
+        n_rows=5,
+        termination_reason_counts={"escaped": 5},
+        n_seed_events=0,
+        device="cpu",
+        torch_threads=1,
+    )
+    assert timing["n_physical_rows"] == 0
+    assert timing["us_per_step"] is None
+    assert timing["write_us_per_step"] is None
+    assert timing["ms_per_event"] is None
+
+
 def test_rollout_exits_1_on_checkpoint_missing_model_config(tmp_path):
     checkpoint = tmp_path / "bad.pt"
     torch.save({"sec_decoder": {}, "normalizer": {"sec_phys": {}}}, checkpoint)