ext/imgmath: batch LaTeX compilation via pre-rendering

Add ``imgmath_batch_size`` config option (default 1). When greater
than 1, equations are collected on ``env-updated`` and compiled into
multi-page DVI documents before writing, reducing N LaTeX processes
to ceil(N/B), then split via ``dvipng -pp`` / ``dvisvgm --page``.

Filenames, embed mode, depth handling, and error reporting match
single-equation mode. Batching is disabled when a custom
``template.tex``/``preview.tex`` template is detected.

Author: jschueller

doc/usage/extensions/math.rst

@@ -190,6 +190,19 @@ are built:
 
    .. versionadded:: 5.2
 
+.. confval:: imgmath_batch_size
+   :type: :code-py:`int`
+   :default: :code-py:`1`
+
+   Number of math expressions to compile in a single LaTeX invocation.
+   When greater than 1, equations are collected after reading and
+   compiled into multi-page DVI documents before writing,
+   reducing *N* LaTeX processes to *ceil(N / batch_size)*.
+   Batching is disabled when a custom ``template.tex``/``preview.tex``
+   template is detected.
+
+   .. versionadded:: 9.2
+
 :mod:`sphinx.ext.mathjax` -- Render math via JavaScript
 -------------------------------------------------------