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.

The error message is unhelpful — it points at a line in the generated texput.tex that you don't have access to (tectonic deletes the temp file). To get useful information:

1. Run pandoc separately with the same template flags to dump the intermediate .tex to a known location:

   pandoc -f markdown+alerts input.md -s -t latex -o /tmp/out.tex \
     --template=assets/cosai-template.tex \
     --lua-filter=assets/callout.lua \
     --pdf-engine=tectonic

2. sed -n '680,695p' /tmp/out.tex (using the line number from the tectonic error) shows the actual offending region. In my case it was clearly \texttt{θ\_t = ...} — Greek letter inside a monospace span.
3. Confirmed by grepping the source markdown for non-ASCII chars (grep -P '[^\x00-\x7F]') and noting that all the failures clustered on lines that contained α/θ/λ INSIDE backticks. The same Greek letters in plain text (no code span) rendered fine because they used the main text font (Montserrat/Computer Modern), which has Greek coverage.

Why "undefined control sequence" instead of a missing-glyph error: when the monospace font has no Greek glyphs, the encoding fallback path produces a sequence that LaTeX interprets as a malformed command, hence the misleading control-sequence error.

Replaced the two offending markdown lines with display math. Rebuilt with the same python convert.py input.md output.pdf command. Exit 0, PDF generated correctly, formulas render in proper math typography on the page. Confirmed by inspecting the rendered PDF (20-page output, formulas appear correctly typeset on pages 11 and 12).