Add MathJax macro for undefined \Square symbol in section 4.1

\Square has no macro definition anywhere in the LyX source (same gap
class as the existing \obslash fix) and is not a real amsmath/amssymb
command. Aliased to \square, the standard hollow-square glyph this
"biunit cube" reference-element notation calls for in eq. 374.

Author: Wesley-Jakob-Gilbert

CONVERSION_NOTES.md

@@ -94,7 +94,7 @@ visible output; see "Specific insets flagged" below.
 
 | Section | Title | Inline formulas | Display formulas | Citations | Figures | Converted cleanly? | Needs manual review |
 |---|---|---|---|---|---|---|---|
-| 4.1 | Solid Elements | 23 | 7 | 1 | 2 | Mostly | 4 `Tabular` insets with merged cells (colspan/rowspan) — not representable in plain Markdown, rendered best-effort and flagged; 2 figures (`FigSolidElementsTM.png`, `FigQuadraticTetrahedralElements.png`) fetched from upstream |
+| 4.1 | Solid Elements | 23 | 7 | 1 | 2 | Mostly | 4 `Tabular` insets with merged cells (colspan/rowspan) — not representable in plain Markdown, rendered best-effort and flagged; 2 figures (`FigSolidElementsTM.png`, `FigQuadraticTetrahedralElements.png`) fetched from upstream; `\Square` (eq. 374, "the biunit cube") has no macro definition anywhere in the source — see the "Specific equations/insets flagged" table below |
 | 4.2 | Shell Elements | 69 | 56 | 11 | 3 | Yes | 3 figures (`FigShellElementsTM.png`, `FigShellElementTypesTM.png`, `FigShellElementsFB.png`) fetched from upstream; equations `eq:virtual-work-internal` and `eq:virtual-work-external`, referenced cross-chapter from Section 3.5, are defined here |
 
 ### Chapter 5 — Constitutive Models
@@ -166,6 +166,7 @@ it) from numeric to lettered numbering and from "Chapter" to "Appendix" in the n
 | 2.1 | eq. (11) `\mbox{\thinspace and\thinspace}` | `\mbox` and `\thinspace` are not in MathJax's default macro set | Added `mbox` and `thinspace` macros to `docs/js/mathjax_config.js`; verified render (see `screenshot_section_2.1.png`) |
 | 2.1, 2.6, 2.9–2.16, 3.x–7.x (throughout) | `\tr`, `\dev`, `\grad`, `\divg`, etc. | Custom operators defined in the *full manual's* LyX preamble (`\newcommand`), not standard LaTeX/MathJax | Added equivalent `macros` entries to `docs/js/mathjax_config.js` |
 | 2.1, 2.6 (eq. 19, 22, 25, 26) | `\obslash` | **No macro definition exists anywhere in the source LyX file for this symbol** — appears to be a gap in the original document, not something this converter introduced | Renders as `⦸` (U+29B8 CIRCLED REVERSE SOLIDUS, `\mathbin{\unicode{x29B8}}` in `mathjax_config.js`), confirmed against the published manual as the correct glyph |
+| 4.1 (eq. 374 and the sentence right after it) | `\Square` | **No macro definition exists anywhere in the source LyX file for this symbol either** — same gap class as `\obslash` above, and confirmed present in the real published manual too, so there's no reference rendering to match against. Per the manual's own prose it denotes "the biunit cube" (the reference/master isoparametric element domain for Gauss-quadrature integration); real amsmath/amssymb only define the lowercase `\square`/`\Box`, not a capitalized `\Square` | Aliased `Square: '\\square'` in `docs/js/mathjax_config.js`, rendering the standard hollow-square glyph (U+25A1) used for this exact "reference element" notation in FEM literature |
 | 2.5, 3.1, 4.1, 4.2, 5.4, 5.6, 5.7, A.1 (throughout) | Figure captions/images | Original image binaries not present in the pilot's original input directory | `build.py` fetches the real artwork from `febiosoftware/FEBio` on GitHub at build time; original LyX caption text preserved verbatim |
 | Chapter-spanning | Cross-section/cross-chapter `\eqref{}`/`\ref{}` to equations, e.g. `\eqref{eq88}` (2.5→2.6), `\eqref{eq:virtual-work-internal}`/`\eqref{eq:virtual-work-external}` (4.2→3.5), `\eqref{eq:ideal-fluid}` (5.16→3.5) | Each Section is a separate page; MathJax's per-page auto-numbering can't resolve a `\label{}` defined on a different page, so these previously rendered as unclickable "???" | `EQ_LABEL_REGISTRY` resolves these to a static link with the target's own page-local equation number, e.g. `(2.5-35)`, linking to MathJax's `#mjx-eqn:<label>` anchor |
 | Chapter-spanning | `\ref{chap:X}` to a chapter (e.g. "see Chapter 5") | Previously rendered the chapter's full title instead of its number, and was unresolvable/inconsistent for chapters not yet converted | `CHAPTER_LABEL_REGISTRY` is populated for every chapter regardless of conversion status; a chapter `\ref{}` now always renders the chapter *number*, hyperlinked if converted, plain text otherwise |

docs/js/mathjax_config.js

@@ -29,6 +29,14 @@ window.MathJax = {
       cay: '\\operatorname{cay}',
       rot: '\\operatorname{rot}',
       obslash: '\\mathbin{\\unicode{x29B8}}',
+      // \Square has no macro definition anywhere in the LyX source either
+      // (same gap class as \obslash above) -- used in section 4.1 eq. 374
+      // and the sentence right after it to denote "the biunit cube" (the
+      // reference/master isoparametric element domain for Gauss-quadrature
+      // integration). Not a real amsmath/amssymb command (which only
+      // define the lowercase \square/\Box), so aliased to \square, the
+      // standard hollow-square glyph for this exact usage in FEM notation.
+      Square: '\\square',
       // \mbox is plain-TeX/LaTeX only; MathJax's default macro set does
       // not define it. The LyX source uses \mbox{...} in a handful of
       // places (section 2.1 eq. 11 "\mbox{\thinspace and\thinspace}";