ci(docs): build API docs from uv.lock instead of a fresh PyPI resolve

[cailmdaley]

The docs job ran uv pip install '.[docs]' inside the image. That re-resolves the whole environment from PyPI, so an unrelated upstream release can fail every PR's docs check. The latest example is shear-psf-leakage 0.2.1 against cs-util 0.2.2's scipy cap.

This PR installs the docs from the lock instead:

• uv sync --frozen --inexact --extra docs installs exactly what the checkout's uv.lock pins into the image's venv, and leaves the rest of the image's stack in place.
• uv pip install --no-deps -e . then installs the checked-out package on top. The project isn't uv-packaged, so without this line autodoc would document the copy baked into the image.
• PRs now build docs only when docs/, src/, pyproject.toml, uv.lock or this workflow change.
• Deployment is unchanged: pushes to develop deploy to Pages.

I checked it locally in the :develop SIF: the sync installs only locked versions and the Sphinx build completes against the checkout. This works together with #362, whichever merges first.

— Claude (Opus) on behalf of Cail

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