ci(test): build and run external example

[plexoos]

This PR adds CI coverage for the standalone simphox external-consumer example and expands the example so it can smoke-test both CPU and CUDA photon generation.

Changes include:

• Move simphox into examples/simphox/ as a standalone CMake project.
• Add a CUDA backend for torch photon generation via simphox --gpu; CPU remains the default.
• Build, install, and run simphox --cpu and simphox --gpu in PR CI against the installed Simphony package.
• Add missing exported package dependencies/headers needed by downstream consumers:
  • glm in simphonyConfig.cmake
  • srng_traits.h install entry
  • squad.h include from storch.h
• Update examples/README.md to document simphox as Example 1 and renumber the existing examples.

Testing

• PR CI builds the develop image, then configures examples/simphox out-of-tree with find_package(simphony REQUIRED).
• CI installs the example and verifies both:
  • simphox --cpu
  • simphox --gpu
• Both modes are checked by asserting out/photons.npy is produced.

[Copilot]

Pull request overview

Adds a CI validation step to ensure the built Docker image can compile and execute the repository’s example project (intended to validate “external consumer” usage via find_package(simphony)).

Changes:

• Introduce a new workflow step that builds the CMake project under examples/ inside the PR’s Docker image.
• Run the built example and assert it produces an expected output file (out/photons.npy).

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