chore(packaging): add free-threaded (cp31Nt) wheels once GDAL SWIG bindings + numpy support no-GIL

[MAfarrag]

Context

The wheel matrix ships GIL builds for CPython 3.11–3.14 across five platforms (manylinux_2_28
x86_64/aarch64 from source, macOS x2, Windows AMD64), with musl canaries on top (#333). Free-threaded
CPython (cp313t / cp314t) is explicitly excluded:

# pyproject.toml [tool.cibuildwheel]
skip = ["*pp*", "*musllinux*", "cp314t-*"]

Status: reactive — blocked on upstream, same model as #335. This issue exists as a tracker so the
free-threaded gap has a number the docs can point at and so the periodic upstream check doesn't get
forgotten.

Related: #332 (from-source Linux stack, shipped), #333 (musl canaries), #334 (win_arm64), #335 (cp315).

Problem / Current Behaviour

Users on a free-threaded interpreter get no wheel: pip falls back to the sdist, which needs a system
GDAL + build chain, and the resulting build still runs with the GIL re-enabled unless every C extension
in the stack declares free-threaded support.

Blockers (all upstream)

1. GDAL SWIG bindings — the osgeo extension modules do not declare Py_mod_gil /
   free-threaded readiness; SWIG-generated code needs auditing for GIL-free safety first.
2. numpy — free-threaded wheels exist but ABI/behaviour is still stabilising across the versions
   pyramids supports.
3. Ecosystem — shapely, pyproj, pandas, geopandas (and pyogrio for vector I/O) all need cp31Nt
   wheels before pip install pyramids-gis would resolve cleanly on a free-threaded interpreter.

When to act

Revisit at the Python 3.15 cycle (when #335 fires): if GDAL's bindings have declared free-threaded
support and numpy/shapely/pyproj ship cp315t wheels, drop the cp314t-*/cp315t-* skip for a spike
build and run the full test matrix against it (both build models: the from-source Linux stack and the
conda-extract macOS/Windows path).

Affected locations

File	Symbol	Notes
pyproject.toml	[tool.cibuildwheel].skip	remove the cp31Nt-* entry
.github/workflows/bundle-pypi-wheels.yml	test-wheels matrix	add a free-threaded cell
docs/installation.md / docs/how-to/wheel-build-flow.md	coverage tables	flip the row when it ships