Add \argmin macro (counterpart to existing \argmax)

[d-morrison]

Closes #73

Adds \argmin (and \amin short alias), the missing counterpart to the existing \am/\argmax. This is a general-purpose operator used across optimization, estimation, and statistics generally — not tied to any one estimand family — so it sits alongside \argmax rather than in a narrower macro group.

Change

\def\amin{\arg \min}
\def\argmin{\arg \min}

Plus matching interpretations.tsv entries, per CONTRIBUTING.md.

Verification

Ran all three commands CONTRIBUTING.md documents before pushing changes to macros.qmd (installed knitr/rmarkdown/DT + TinyTeX locally to do this for real, since CI's "Quarto Preview" workflow only runs a default quarto render, which doesn't cover the two PDF-specific commands):

• quarto render demo-include-in-header.qmd --to pdf — succeeds, _site/demo-include-in-header.pdf produced
• quarto render demo-shortcode.qmd --to pdf — succeeds, _site/demo-shortcode.pdf produced
• quarto render (whole site) — succeeds, including macros-table.qmd's interpretations-consistency check (640 macros parsed, zero missing interpretations.tsv entries); \amin/\argmin both appear correctly in the built macro reference table

No new macro-name collisions (the only duplicate names in the corpus — \resid, \stdresid, \modresid — are pre-existing and unrelated to this change).

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