Always serialize editor scripts with braces

[charlesroddie]

The editor's LaTeX serializer currently removes braces from one-atom scripts, producing x^2 and \log_2(x), while multi-atom scripts necessarily use braces. Use one canonical form for both cases: x^{2} and \log_{2}(x).

Acceptance criteria:

• The parser continues to accept braced and unbraced one-token scripts.
• MathListToLaTeX always emits braces around subscripts and superscripts, including a single atom.
• MathKeyboard/editor output uses the same canonical form.
• Round-trip and editor keypress tests cover single-token, multi-token, nested, and empty/placeholder scripts.
• Document the intentional serialized-string change for consumers comparing LaTeX text.