[cursor]

Problem

latex_math_to_plain recursed through convert() for every nested \frac, ^, _, \sqrt, and \text with no depth limit. TUI and HTTP markdown both call this on model and user text. A few hundred nested groups — easy to emit from a model or paste — can overflow an 8 MiB worker thread stack and kill the process. The JSON parser already caps nesting for the same reason (json.cpp::kMaxDepth).

Fix

Thread a depth counter through convert / convert_script. After 64 levels, emit the remaining raw fragment instead of recursing. 64 is deeper than any legitimate equation and far below the stack limit. Shallow nested fractions and scripts still convert as before.

Tests

unit_markdown_diff:

• Modest nested \frac{\frac{a}{b}}{c} and a^{b^{2}} still convert.
• 400 nested \frac groups return a finite string (slash form on the converted prefix, raw \frac past the cap) instead of overflowing.
• 400 nested superscripts and a \[...\] display-math path also survive.

Suite 50/50 locally (281 assertions) + ASan + UBSan. arbiter compiles.

Independently mergeable against main (0456350). Does not touch files owned by #321–#356 except CHANGELOG [Unreleased], which sibling-conflicts after the first of those merges, same as the rest of the series.

  

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Note

Low Risk
Localized defensive change to LaTeX-to-plain conversion with a documented depth cap; normal equations unchanged, only extreme nesting degrades to raw fragments instead of crashing.

Overview
Fixes a stack-overflow risk when the TUI/API render LaTeX in markdown: latex_math_to_plain now tracks recursion depth and stops converting after 64 nested \frac, ^, _, \sqrt, or \text groups, emitting the rest as raw TeX instead of recursing further.

Depth is threaded through internal convert / convert_script calls (same entry point as before for callers). Shallow math still converts to Unicode as today; pathological model/user input no longer blows the worker thread stack. Header and [Unreleased] changelog document the cap.

Tests add modest nested fraction/script cases, 400-level \frac and superscript nests (finite output with raw \frac past the cap), and a MarkdownRenderer \[...\] path over deeply nested math.

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