Freeze maths inside list items so the delimiter checks pass

[peterbjohnson]

Six of the nine PHYS40002 tex sheets fault on maths delimiters after t44, all in list items. Two causes, seen in the frozen markdown of a real sweep:

1. A display maths that pandoc writes over several lines inside a list item - ... given by $$U = 5 \times 10^{-21} \left( ... \right) then a continuation line indented past the item's content column, ending \text{J},$$ where $r$ is ... - is left as written, because _verbatim_lines counts the continuation line (indented four or more past the item's column) as a code block and _display_maths_blocked then skips the match. A line inside an open $$ ... $$ is never code: track open display maths while walking the lines, and treat every line of the span as maths. The rewrite then puts the $$ on lines of their own, indented to the item, with the words before and after as their own paragraphs, as it does for a one-line match. Findings this removes: opening $$ must be followed by a newline, opening $$ must start its own line, closing $$ must start its own line.
2. An inline $ ... $ whose author broke the line inside it - $\frac{1}{2} m v^2 = 576\,$ written over two lines in the .tex - freezes with the newline inside, and the checks report it as display $$ ... $$ closed with a single $ or newline inside an inline $ ... $. A newline inside inline maths carries nothing: join the lines of an inline $ ... $ span with a space at the freeze, in a paragraph or a list item, before the display-maths rewrite runs, so that the check sees one line. Do not touch $$ spans here.
   Fixtures: add both shapes to tests/fixtures/sources/display_maths/source.tex (a display maths over two lines inside an item with words either side; an inline maths broken over two lines) and re-freeze its expected.json; tests/test_source.py's delimiter assertion over every block then covers them. Done when the two shapes freeze to fields math_delimiter_checker passes, and the existing fixtures re-freeze unchanged except display_maths.

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Workbench ticket t56.