Add ring / asterisk / circled-times operators and math-symbol samples

- \circ (U+2218): reuses the ring contours extracted from Å (same
  hand-drawn shape used as a diacritic above å/ů), recentred on the
  math axis.
- \ast (U+2217): asterisk outlines translated down to the math axis.
- \otimes (U+2297): hand-drawn from the 2034 equations comic via
  extras/2034_equations_2x__circled_times.png.
- Use createChar(cp, name) instead of createMappedChar(cp) when adding
  Unicode-mapped glyphs in pt6 — createMappedChar leaves unicode=-1 in
  the SFD and the OTF cmap omits the codepoint (which is why \cdot
  had been silently falling back to MJXTEX since it was introduced).
- Samples: align-eqref (AMS align + tag + eqref, opts into the
  [tex]/ams package via its `mathjax:` block) and math-symbols-zoo
  (binops / relations / arrows / delimiters / logic / misc reference).

Author: pelson

xkcd-script/samples/mathjax3/accents-zoo.png

@@ -78,10 +78,6 @@ deeply-nested-sqrt:
   formulas:
     - tex: '$$\sqrt{\frac{b^2}{\frac{b^2}{\frac{b^2}{\frac{b^2}{4ac}}}}}$$'
 
-sqrt-pi-short:
-  formulas:
-    - tex: '$\sqrt{\pi}$'
-
 piecewise-single:
   formulas:
     - tex: '$$y = \begin{cases} x & \text{if } x > 0 \end{cases}$$'
@@ -161,6 +157,7 @@ sqrt-digit-radicand:
   cellHeight: 80
   cellWidth: 280
   formulas:
+    - tex: '$\sqrt{\pi}$'
     - tex: '$\sqrt{5}$'
     - tex: '$\sqrt{2}$'
     - tex: '$\sqrt{1-x^2}$'
@@ -179,3 +176,37 @@ maxwell-equations:
 ramanujan-identity:
   formulas:
     - tex: '$$\frac{1}{(\sqrt{\phi \sqrt{5}}-\phi) e^{\frac25 \pi}} = 1 + \frac{e^{-2\pi}}{1 + \frac{e^{-4\pi}}{1 + \frac{e^{-6\pi}}{1 + \frac{e^{-8\pi}}{1 + \cdots}}}}$$'
+
+# Exercises the AMS `align` environment with multi-column alignment
+# (`& = … &&`), \tag/\label/\eqref cross-references, and the new math
+# operators \circ (U+2218) and `*` / \ast (U+2217).
+align-eqref:
+  mathjax: { packages: [ams], tags: ams }
+  formulas:
+    - tex: |
+        $$\begin{align}
+        y_1 + y_2 &= z \tag{$\star$} \label{ref1} \\
+        y' \circ y^* &= w \tag{$\dagger$} \label{ref3}
+        \end{align}$$
+
+        $$\begin{align}
+        x & = y_1 - y_2 + y_3 - y_5 + y_8 - \dots && \text{by \eqref{ref1}} \\
+          & = y' \circ y^*                         && \text{(by \eqref{ref3})} \\
+          & = y(0)\, y'                            && \text{by Axiom 1.}
+        \end{align}$$
+
+# A reference grid of math operators, relations, arrows, delimiters,
+# logic, and miscellaneous symbols that realistically appear in TeX
+# equations.  Useful both as a visual reference for what xkcd-script
+# covers and as a gap-finder — any cell that renders as a fallback
+# (Times-style) glyph or a `.notdef` box points at a missing codepoint.
+math-symbols-zoo:
+  cellHeight: 64
+  cellWidth: 700
+  formulas:
+    - tex: 'Binary ops: $+\ -\ \pm\ \mp\ \times\ \div\ \cdot\ \ast\ \circ\ \bullet\ \oplus\ \ominus\ \otimes\ \odot\ \cap\ \cup$'
+    - tex: 'Relations: $=\ \neq\ <\ >\ \leq\ \geq\ \ll\ \gg\ \approx\ \equiv\ \sim\ \simeq\ \propto\ \in\ \notin\ \subset\ \subseteq$'
+    - tex: 'Arrows: $\to\ \gets\ \rightarrow\ \leftarrow\ \Rightarrow\ \Leftarrow\ \Leftrightarrow\ \mapsto\ \uparrow\ \downarrow$'
+    - tex: 'Delimiters: $(\ )\ [\ ]\ \{\ \}\ \langle\ \rangle\ |\ \|\ \lfloor\ \rfloor\ \lceil\ \rceil$'
+    - tex: 'Logic / set: $\land\ \lor\ \lnot\ \implies\ \iff\ \forall\ \exists\ \emptyset\ \in\ \subset\ \cup\ \cap$'
+    - tex: 'Misc: $\infty\ \nabla\ \partial\ \aleph\ \hbar\ \ell\ \Re\ \Im\ \wp\ \prime\ \dagger\ \ddagger\ \star\ \angle$'