Decouple SpheriCart from Polynomials4ML

[cortner]

Decouples P4ML from SpheriCart. The spherical-harmonics wrapper and the
atomic-orbitals basis (P4ML's only SpheriCart consumer) move out, so P4ML
core no longer depends on SpheriCart at all.

Changes

• Remove sphericart (src/sphericart.jl deleted; SpheriCart dropped
  from [deps]/[compat]). The harmonics + their evaluate interface now
  live in SpheriCart itself (Complex harmonics + ACEbase evaluate-interface extension lab-cosmo/sphericart#235).
• natural_indices via ACEbase: import ACEbase: natural_indices
  instead of a local stub, so it is the shared generic
  (Add natural_indices to the shared evaluation interface ACEbase.jl#10). ACEbase compat → 0.4.7.
• Extract AtomicOrbitals into a new lib/AtomicOrbitals subpackage
  (mirrors EquivariantTensors lib/ACEradials; not a P4ML dependency).
  Its angular Ylm is now a bare SpheriCart basis used purely via the
  ACEbase interface (parameter/state-free, Bumper-stack buffers keep it
  allocation-free). All dead code (commented Lux layers / hyperdual /
  evaluate_ed_dp blocks) dropped in the move.
• Tests updated/relocated (test_sphericart.jl, test_atorbrad.jl removed
  from core; sphericart cases dropped from test_gpu/test_lux/
  test_transformed).

Breaking (version 0.5.10 → 0.6.0):
Polynomials4ML.real_solidharmonics / real_sphericalharmonics /
complex_* / AtomicOrbitals are gone. Downstream should use SpheriCart

• ACEbase for harmonics, or lib/AtomicOrbitals for atomic-orbital bases.

Depends on ACEbase ≥ 0.4.7 (#10) and SpheriCart ≥ 0.2.4 (#235);
CI here will only resolve once those are registered.

Verified locally (deps dev'd): P4ML Pkg.test 1303/1303;
lib/AtomicOrbitals Pkg.test 141/141 (zero allocations).

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[cortner]

Update: AtomicOrbitals kept in P4ML via a SpheriCart extension (not a separate lib)

Reconsidered the lib/AtomicOrbitals extraction — a generically-named, registerable AtomicOrbitals.jl implementing only a thin ACE-specific slice would squat a prime general-purpose name. Reverted that part and kept AtomicOrbitals inside P4ML, with the SpheriCart-specific glue in a package extension.

• AtomicOrbitals + RadialDecay/GaussianDecay/SlaterDecay and all Ylm-generic evaluation/pullback/Lux logic now live in src/atomicorbitals/ and are exported from P4ML again.
• The only SpheriCart-typed code — the complex _ylm_valtype specialization and the _rand_* fixtures — lives in ext/SpheriCartExt.jl, loaded via [weakdeps] SpheriCart. No hard runtime dep; SpheriCart is a weakdep + test-dep only.
• AtomicOrbitals is generic over any ACEbase-evaluate Ylm: a real fallback _ylm_valtype keeps the type usable without SpheriCart, and the extension supplies the precise complex value type.
• lib/AtomicOrbitals/ removed; its test moved to test/test_atorbrad.jl.

This revises the earlier "no [deps]/[weakdeps] on SpheriCart" stance — a weak dependency is now allowed. P4ML still carries no SCWrapper/src/sphericart.jl code.

Pkg.test("Polynomials4ML") is green (1444 tests, incl. the Atomic Orbitals set); SpheriCartExt precompiles and loads.

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[cortner]

Follow-up (review fixes):

• Restored the source file name src/atomicorbitals/atomicorbitals.jl (the lib had renamed it to aorbasis.jl).
• Moved the _rand_gaussian_basis/_rand_slater_basis/_rand_sto_basis (and _rand_basis) example builders into P4ML core. The single SpheriCart touchpoint — the default SolidHarmonics angular basis — is routed through a small _default_ylm hook (stub in core, method in ext/SpheriCartExt.jl), so the builders stay in core but still need import SpheriCart.
• Re-added the three atomic-orbital cases to test/test_lux.jl.
• Documented the usage: an AtomicOrbitals docstring stating you must import SpheriCart first, plus an entry in docs/src/polynomials.md.
• Fixed the docs build: dropped stale @refs to removed harmonics functions, de-linked the now-ACEbase-owned natural_indices, and updated the regression tutorial from real_sphericalharmonics to SpheriCart.SphericalHarmonics (added SpheriCart to docs/Project.toml).

Verified: Pkg.test("Polynomials4ML") → 1459/1459 pass; docs/make.jl builds cleanly.

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