Add shared Categories list-of-categories datastructure

[cortner]

Summary

Adds a single, generic, isbits list-of-categories datastructure to ACEbase,
consolidating the zlist / _i2z / _z2i machinery that ACEpotentials, ACEradials,
and AtomicOrbitalKernels each reimplement. New file src/categories.jl (plus tests);
no new dependencies; not exported (use ACEbase.Categories, ACEbase.cat2idx, …).
Version stays 0.4.7 — this is a non-breaking addition.

Design

• Categories{N,T} stores a sorted SVector{N,T} of distinct categories; the sorted
  position is the 1-based index. Generic over the category type T (chemical species =
  T=Int atomic numbers); isbits whenever T is.
• cat2idx(c, z) — category → index — is non-throwing and allocation-free (returns
  0 when absent), so it is safe inside GPU kernels. Compile-time branch on N: linear
  scan for small N (≤ 8), binary search (searchsortedfirst) for many species.
• idx2cat(c, i) — index → category. Collection interface (length, iterate,
  getindex, eltype, in, ==, show). Batched lookup via broadcast:
  cat2idx.(c, zs) (a Base.broadcastable method treats a Categories as a scalar).
• Constructor Categories(itr; allow_nonbits=false) sorts, checks distinctness, and
  requires isbitstype(T) by default (so the struct is always device-movable);
  allow_nonbits=true opts out for CPU-only / abstract-category use (e.g. Symbol).
  No AtomsBase dependency — chemical-symbol → atomic-number conversion stays in the
  consumer.
• Related helpers in scope: make_smatrix(obj, NZ) (per-pair parameter matrices, with a
  dense-Matrix fallback for large NZ) and the pair-index utilities symidx,
  catcat2idx, catcat2idx_sym.

Notes / follow-ups

• Sorted-order caveat: indices are defined by sorted order rather than insertion
  order; when a consumer migrates, its per-species/per-pair parameter arrays must be
  addressed consistently through idx2cat/cat2idx, and any pre-trained models would
  need their species axes permuted on load.
• Downstream migration (ACEradials first) is out of scope here.

Testing

Pkg.test() — 634/634 pass. Covers the bijection round-trip (Int and Char), the
sorted invariant, both lookup regimes (N=3 and N=100), absent-key handling, the
isbits policy + opt-out, allocation-free lookup, broadcast lookup, and the pair
utilities / make_smatrix fallback.

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