Add pixi support for development environment and installation

[jaredgalloway]

Warning

STUB — not ready to be worked on. This issue captures a goal only;
it has not been through brainstorming or specification. Its body will
be replaced with a full spec before any work begins. Do not plan
or implement against this text.

Add pixi as a supported dependency-management and environment tool for
phippery. Contributors should be able to run pixi install / pixi run pytest against a committed pixi.lock instead of hand-rolling a venv,
with the scientific stack (numpy, scipy, xarray, statsmodels,
biopython, POT) sourced from conda-forge where reliable.

Existing PEP 621 [project] metadata and PyPI publishing stay intact —
this adds a [tool.pixi.*] surface, not a rewrite. Scope of dev-env
vs. end-user install path to be settled during /spec.