#662 setup: prefer uv for the venv and package installs when uv is installed

Description

On NixOS, a venv made with the system Python cannot use the pip wheels
Strata needs (numpy/pillow dlopen libraries outside the store, and the
system interpreter is not one the wheels are built against). `uv` with
its own managed interpreter works there, and `uv pip` is faster
everywhere else.

When `uv` is on `PATH`:

- `setup.sh` creates `.venv` with `uv venv --managed-python --seed
  --python 3.12` — uv's own interpreter, which finds its libraries
  without help from the distro.
- `setup.py`'s package step installs with `uv pip install --python
  <sys.executable>`.

Without `uv`, every path is exactly as before (standard `venv` + pip,
same package list, same stamps). Windows is untouched.

This makes the NixOS path documented in `docs/DETAILS.md` work with the
distro's own package manager: `uv` in `environment.systemPackages`,
nothing else.

Tested on NixOS (unstable, RTX + CUDA): fresh `./setup.sh` creates the
uv venv, installs the pinned packages, compiles the engine, starts the
model; a second run starts directly. On a Fedora box without uv:
unchanged behavior.