Add scalar Complex boxing/unboxing across the jl2py ABI

[jack-champagne]

Scalar Complex was the one input/output gap in the otherwise-complete bridge
(arrays of complex already worked). For a quantum-control codebase (Piccolo's
PAULIS/GATES/Hamiltonians are complex) this is first-class, so:

Input — box_python_value boxes a Python complex → Julia Complex{Float64}
via new C jl2py_box_complex128 (Base.complex(re, im)).

Output — compute_typetag now emits TAG_COMPLEX128/TAG_COMPLEX64 for scalar
Complex{Float64}/{Float32} (cached at init), and new JuliaComplex128/JuliaComplex64
wrappers expose __complex__ / .real / .imag via new C jl2py_unbox_complex128.

Before: a Python complex arg raised TypeError; a scalar Complex result fell
through to bare JuliaAny (no .real/__complex__; .re/.im worked but un-Pythonic).

Verified locally on Julia 1.12.6 (libjl2py rebuilt clean):

• existing int/float/str primitives unchanged (regression);
• complex in/out round-trips; ComplexF32 promoted; complex as a kwarg value;
• and through a SOURCE-FREE package-image-loaded function (via JuliaStaticData) —
  scalar complex from a runtime-loaded module tags + wraps correctly with no world-age wall.

Adds tests/test_complex.py (numpy-free; mirrors the existing regression-test style).

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