Fix scalar solver handling in the Clothoid path planner

[Afloat16]

What does this implement/fix?

The Clothoid planner passes fsolve's one-element input array directly into scalar quadrature and trigonometry, then retains the solver's array result. With the project's current NumPy pins, valid paths return None after a scalar-conversion error. The existing animation-free smoke test still passes because it does not inspect the generated paths.

Extract the scalar fitting parameter at the solver boundary. Read the solver's diagnostics and require a finite root and residual. A stalled result is accepted only when the dimensionless residual is within double-precision roundoff; this preserves analytic circular paths whose exact root is zero under the existing warnings-as-errors test policy. Larger failed-fit residuals remain errors.

The initial guess, fitting equations, public path inputs, units and point-list contract are unchanged. The existing official RST equations and explanation remain correct, so this code-only correction adds no documentation, figure or animation change.

Validation

• Exact base: cdd0cc888802b584c2d654ca85e3c5460973487d.
• The identical new tests on the base: 16 failed, 11 passed. With this patch: 27 passed under -Werror.
• Independent straight-line and signed circular-arc coordinates cover scale, rotation and translation. Public multi-orientation generation and the complete 75-orientation default grid produce finite complete paths with correct endpoints.
• Nonfinite start/goal orientations and a zero-length segment remain rejected. Scalar integration and circular-parameter controls remain correct.
• bash runtests.sh: 182 passed on Linux/Python 3.14.8 with the exact requirements, including the configured diff-Ruff and mypy checks. No skips or deselections were added.
• Strict Sphinx HTML build: passed. The documented Clothoid page has unchanged text, equations, links, images and API content; the only generated difference is a cache-version query for an identical existing CSS asset. Browser-rendered HTML is not claimed.
• Other operating systems and Python versions were not tested locally. No physical robot, animation export or performance claim is made.

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