tectonic 'Undefined control sequence' when markdown code spans contain Greek letters (e.g. θ, α)

When converting a markdown document to PDF via pandoc + tectonic (CoSAI whitepaper converter / similar pipelines), tectonic fails with a cryptic error like:

error: texput.tex:685: Undefined control sequence
error: halted on potentially-recoverable error as specified
Error producing PDF.

The line number points into a generated intermediate .tex file that the user never wrote. Conversion succeeds for almost the entire document, then breaks at one specific spot. The error message gives no hint about what character or markup is responsible.

In my case the markdown contained a formula written as an inline code span:

> `θ_t = θ_base × (1 + scaling × surprise_t)`

Pandoc converts that to:

\texttt{θ\_t = θ\_base × (1 + scaling × surprise\_t)}

and tectonic chokes on the Greek θ inside \texttt{} because the default monospace font (Latin Modern Mono / LMTT) does not contain glyphs for Greek letters. Even though tectonic has full Unicode support generally, the specific monospace font selected for code spans does not, and LaTeX surfaces the missing-glyph as "undefined control sequence" rather than as a font-coverage error.