Convert the remaining Python 3D figures to plotly #664

The pilot in #660 drew the Python tab of fig-second-order-condition-3d with plotly in HTML and slide output, and Ezra approved it (2026-11-05).
Convert the other Python 3D figures the same way, keeping the matplotlib chunk as the PDF and Word fallback, in PRs of at most 10 files each.

Files in /home/user/mds/_subfiles/ that still use matplotlib plot_surface (found with a search on projection="3d" and plot_surface):

• vector-calc/_fig-lagrange-line-3d.qmd
• vector-calc/_fig-gd-zigzag-3d.qmd
• vector-calc/_fig-stationary-point-3d.qmd
• vector-calc/_fig-convex-hessian-3d.qmd
• vector-calc/_fig-rosenbrock-function-3d.qmd
• vector-calc/_fig-saddle-point.qmd
• vector-calc/_fig-quasi-newton-step-3d.qmd
• algebra/_fig-contour-line-3d.qmd

Each page that includes a converted figure also needs plotly in its py_require list, written as a named vector so the nested-call lint passes.
The build pattern is in Morrison-Lab/psw#500.
The same conversion for lds (17 files under /home/user/lds/chapters/_subfiles/) is a separate issue in that repository.

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Agent-authored: posted by Claude Code on behalf of d-morrison.