feat: allow opt-in for new backends

[b-long]

⚠️ work-in-progress ⚠️

Relates-to: #408

Adds opt-in alternatives to pybindgen, chosen with GOPY_BACKEND. The default stays pybindgen, so nothing changes unless you opt in.

Backend	Needs	Status
cffi	cffi	full suite passes
pybind11	pybind11, C++ compiler	full suite passes
nanobind	nanobind, C++ compiler	full suite passes
capi	nothing beyond Python	full suite passes
cgo	—	not started

Each backend has its own CI job on Linux, macOS and Windows (Python 3.11 and 3.12), with pybindgen not installed. A benchmark job compares per-call time and memory across all backends.

Notes

• capi keeps the default backend's Go shim and build, but writes the C module itself instead of calling pybindgen. Its output matches pybindgen's argument handling and errors. It's
  the simplest way to drop the pybindgen dependency.
• cffi, pybind11 and nanobind share a Go shim that avoids the CPython C API. The Python side is either a cffi module or a compiled C++ module.
• TestMakefile is still skipped for pybindgen and capi. Their shared Makefile links against a separate Go library that isn't found at import time. This problem predates this PR.

To do

• Decide what cgo should mean, or drop it (it overlaps with capi)
• Fix the pybindgen/capi Makefile, then stop skipping TestMakefile for them