Fix B-spline curvature and preserve finite-scale paths

[Afloat16]

Reference issue

The B-spline curvature formula was introduced in #720. Current B-spline and spline-curvature searches found no existing repair; the older cubic-spline issue #64 concerns a separate implementation.

What does this implement/fix?

The shared B-spline evaluator raises squared tangent length to 2/3 instead of 3/2. Returned curvature depends on parameter speed and has incorrect physical units. For the exact parabola x(u)=u, y(u)=u², curvature at u=1 is currently 0.683990 instead of the analytic 0.178885. Doubling a public interpolation path multiplies curvature by 1.587401; it should halve it.

Evaluate signed planar curvature using hypot for tangent length, a normalized tangent, and successive divisions by its length. This is algebraically (x′y″−y′x″)/(x′²+y′²)^(3/2) while avoiding cubed-length underflow for a finite straight path scaled by 1e-110. Public positions and headings are preserved. Both public docstrings now describe inverse-position units, and the existing official B-spline page explains the equation, sign, scale invariance and undefined zero-tangent case.

Regressions use closed-form positive and negative parabolas at three parameter speeds, interpolation and smoothing at degrees 2–5 under two spatial scales, and straight-path controls. Additional straight and signed-parabola cases cover finite coordinate scales 1e-110 and 1e110.

Additional information

• Exact base: cdd0cc888802b584c2d654ca85e3c5460973487d.
• Linux/Python 3.14.8 with the repository's exact pinned test requirements: identical B-spline tests give 26 failures and 9 controls on the base, then 35 passes on this patch under -Werror.
• Complete required bash runtests.sh: 188 passed, including the actual changed-file Ruff and all nine configured mypy scopes. No skip or deselection was added.
• Ruff on the two changed Python files and git diff --check pass.
• The default example runs with the Agg backend; both approximation and interpolation plots were generated and visually inspected.
• CircleCI-style strict make html passes on Python 3.13.16 with the exact project and documentation requirements. The exact-base documentation build also passed. Generated equation markup, both function source links, local images and parent navigation were checked.
• Rendered HTML pixel QA is pending the official PR documentation artifact. This remains a draft while the documented ci/circleci: build_doc / artifact stage runs and its changed page can be inspected. Local HTML rendering was unavailable, and no browser-security, file-protocol or hosting workaround was used. A successful build is not claimed as rendered-page QA.
• Windows/macOS, upstream CI and physical robots were not tested locally. Curvature remains undefined at a zero tangent.

AI assistance: prepared with OpenAI Codex (gpt-6.1-sol). The native validation above was executed in the current environment.

CheckList

• Did you add an unittest for your new example or defect fix?
• Did you add documents for your new example? The existing official B-spline page and public docstrings were updated; rendered HTML QA is pending the official artifact.
• All CIs are green? Draft CI and official artifact QA remain pending.