Add dRICH coverage for photon transport through the aerogel transmission stack

[plexoos]

The existing dRICH simg4ox integration test launches photons directly at a SiPM patch. It therefore does not exercise the realistic upstream radiator path: aerogel → air gap → filter.

This leaves touching sibling-volume boundaries in the dRICH geometry unvalidated, despite their importance to optical-photon transport. Regressions in boundary handling could pass the current direct-sensor test while preventing photons from traversing the radiator stack.

Details

Here the "aerogel transmission stack" or "radiator stack" means the three coaxial dRICH volumes a photon encounters downstream:

Aerogel_DRICH → AirOptical gap → Acrylic_DRICH filter → surrounding C2F6_DRICH gas

More concretely:

• Aerogel: the actual Cherenkov radiator, 80 mm thick.
• Air gap: a 0.4 mm spacer.
• Filter: a 6 mm acrylic layer.
• The test checks transmission at the three downstream interfaces: aerogel→gap, gap→filter, and filter→gas.

Acceptance criteria

• Add an integration scenario that starts photons inside the aerogel near its downstream face.
• Record and validate ordered transmission across the aerogel/air-gap, air-gap/filter, and filter-exit boundaries.
• Run the validation for both Geant4 and Opticks.
• Keep the existing direct-SiPM hit test as complementary coverage.**