XSL: drop deprecated "jsxgraph" and "paragraphs" as "sidebyside" panels

[rbeezer]

Summary

A "jsxgraph" and a "paragraphs" have been deprecated as panels of a "sidebyside" since 2018 — "jsxgraph" was absorbed into "interactive", and "paragraphs" was reserved as a lightweight division. The schema already disallows both in that position (a "paragraphs" is not a valid "sidebyside" panel, and "jsxgraph" is not permitted anywhere), yet the stylesheets still rendered them: the shared panel-content entity SBSPANEL, introduced with the side-by-side entity work in #3057, continued to list them.

This removes the two elements from SBSPANEL, so a "jsxgraph" or "paragraphs" placed directly in a "sidebyside" is no longer rendered as a panel. Because that single entity drives the panel selection in the common "sidebyside" template, the change applies uniformly to HTML, LaTeX/PDF, and PDF via FO. The deprecation warnings are updated to say so.

Changes

• xsl/entities.ent: drop "jsxgraph" and "paragraphs" from SBSPANEL, and trim the accompanying comment. STACKABLE already excluded both, so a "stack" panel is unaffected.
• xsl/pretext-common.xsl: add a "sidebyside"-specific deprecation for "jsxgraph" (mirroring the existing "paragraphs" one), and extend both messages to note that, as of 2026-07-22, such a panel and its content are silently ignored.

Notes

• A standalone "jsxgraph" (outside a "sidebyside") is untouched — its general deprecation message is unchanged and it continues to render.
• Verified: a "sidebyside" containing a "paragraphs" or "jsxgraph" panel now renders neither in HTML, LaTeX, or FO, while both deprecation warnings still fire. The sample article — whose "jsxgraph" and "paragraphs" are all standalone or divisions — is unchanged.

Claude Opus 4.8, acting as a coding assistant for Rob Beezer