Boost pyoptex optimal-design test coverage + reliance assessment

[kgdunn]

What and why

The optimal-design code (experiments/designs_optimal.py) integrates the optional pyoptex library for D/I/A-optimal and split-plot designs. Two gaps:

1. The pyoptex-gated tests silently skip in CI. pyoptex is not declared anywhere in pyproject.toml, so CI's uv sync --all-extras never installs it. Every @_skip_no_pyoptex test is skipped on every run.
2. Coverage gaps in the fallback / adapter / dispatch branches and in the pyoptex path (categorical factors, multiple hard-to-change factors, model-type variants).

This PR boosts coverage.

Key finding (shapes the approach)

The trigger for this work, upstream PR mborn1/pyoptex#49 (loosen plotly/pandas/scikit-learn pins), is merged but not released to PyPI. The latest release pyoptex==1.2.1 still pins plotly~=5.24 (<6), which conflicts with this repo's plotly>=6.5.2 (verified: pip resolution fails). So pyoptex cannot be added to the --all-extras resolution from PyPI today without breaking CI.

We therefore keep pyoptex optional (the architecture already degrades cleanly: D-optimal has a point-exchange fallback; I/A-optimal raise a clear ImportError) and get real test signal from an isolated, non-blocking CI job that installs pyoptex from git main.

Changes

• CI: new non-blocking test-with-pyoptex job that installs pyoptex from git main and runs the optimal-design tests, so the gated paths get exercised without coupling core resolution to an unreleased upstream.
• Tests (no pyoptex needed): fallback edge cases - SEC-19 candidate-set cap, n_points >= k+1 floor, budget capping, model-type passthrough.
• Tests (pyoptex path): categorical-factor conversion, multiple hard-to-change factors, model-type variants, a direct unit test of _convert_factors_to_pyoptex, and D/I/A criterion differentiation.
• Docs: a "reliance on pyoptex" assessment in docs/doe/coverage.md - optional by design, BSD-3 (vendoring possible but a poor trade given the Cython engine), infrequent releases, exit criterion for promoting it to a declared dependency.
• Version/changelog/citation bump per the repo conventions.

https://claude.ai/code/session_01RmvzhdRZ7K2WDDxtu5ej81

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