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>