g4adept: new package

[wdconinc]

This PR adds g4adept, v0.2.0. This is an accelerated demonstrator of electromagnetic particle transport that builds on g4hepem.

Needs:

• g4hepem: new package #4149
• veccore: add cuda support (ahem) #4151 (not needed)

Test build:

-- linux-ubuntu24.04-skylake_avx512 / %c,cxx=gcc@14.2.0 ---------
tde4urx g4adept@0.3.0+covfie+cuda+examples~ipo~mixed_precision~split_kernels~surf build_system=cmake build_type=Release cuda_arch:=75 generator=make
==> 1 installed package

[Copilot]

Pull request overview

Adds a new Spack package recipe for g4adept v0.2.0 (CUDA-accelerated EM particle transport demonstrator) to the builtin package repository, intended to build against external Geant4/VecGeom/G4VG/G4HepEm dependencies.

Changes:

• Introduces repos/spack_repo/builtin/packages/g4adept/package.py with versions, variants, dependencies, and CMake configuration options.
• Enforces CUDA as a required build feature via a ~cuda conflict.

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[wdconinc]

@sethrj This builds now, but I have not gotten around to testing.

[sethrj]

LGTM modulo these couple of comments.

[wdconinc]

This PR allows for successful inclusion of AdePT in DD4hep simulations (AIDASoft/DD4hep#1606, WIP), with some remaining work similar to the celeritas integration.