Complex harmonics + ACEbase evaluate-interface extension

[cortner]

Part of decoupling the spherical-harmonics interface from Polynomials4ML
(P4ML): the standard evaluate/evaluate_ed interface to spherical
harmonics should be available from SpheriCart + ACEbase alone, with no
P4ML dependency. Functionality moved here from P4ML's src/sphericart.jl.

Changes (Julia bindings)

• Complex harmonics in core (src/complex.jl): new
  ComplexSolidHarmonics / ComplexSphericalHarmonics types wrapping the
  real bases, with compute/compute!/compute_with_gradients[!] producing
  complex output via the standard real→complex (±m) linear combination
  (ported from P4ML's _convert_R2C!). Adds Base.length for all four
  harmonics types.
• ACEbase extension (ext/ACEbaseExt.jl, trigger: ACEbase): provides
  evaluate / evaluate_ed / evaluate! / evaluate_ed! / natural_indices
  for all four harmonics types by forwarding to the native compute API
  (GPU support inherited automatically from the existing compute! methods).
• Standalone tests (test/test_aceinterface.jl): complex-vs-reference,
  complex == R2C(real), ACEbase-interface == compute, natural_indices,
  and finite-difference gradients. No P4ML test dependency.

CI / test-infrastructure

• New dedicated Julia CI (.github/workflows/julia-tests.yml): runs the
  julia/ package test suite across Julia 1.10 / 1.11 / 1.12 on
  ubuntu-latest, scoped to julia/** so it doesn't run on pure C++/Python PRs.
• GPU backend auto-detection in the KA tests (test/utils_gpu.jl,
  mechanism adapted from EquivariantTensors): replaces the previously
  hard-coded using CUDA, Metal, AMDGPU + forced gpu = MtlArray in
  test_ka.jl. The system is probed without loading any GPU package, so a
  plain CI runner resolves to CPU (dev = identity) and installs nothing;
  on a real GPU the matching backend (CUDA/Metal/AMDGPU/oneAPI) is installed
  on demand into the sandboxed test env and the device KA tests run. Override
  with TEST_BACKEND. The GPU backends are no longer test dependencies.

Non-breaking; version 0.2.3 → 0.2.4.

Verified locally: Pkg.test green both with TEST_BACKEND=CPU (840 tests,
device KA tests skipped) and auto-detected Metal on Apple Silicon (856 tests,
device KA tests run on the GPU).

Depends on ACEbase ≥ 0.4.7 (adds the natural_indices stub):
ACEsuit/ACEbase.jl#10 (merged; awaiting registration). CI here will only
resolve once that is registered in General.

🤖 Generated with Claude Code