Fix locale-sensitive post_mod coordinate parsing

[dmerkle]

Summary

Fix PostMØD's Graphviz .plain parsing so coordinate dimensions are interpreted with the C locale.

Under locales such as de_DE.UTF-8, the size check for Graphviz output can misclassify small figures as too large. For example, a graph line like graph 1 3.1224 3.2197 can trigger the \dontUseTooLargeCoords fallback, causing summaries to show Figure too large even though the DG is small enough to embed.

This change runs the relevant awk parsers with LC_ALL=C while leaving the surrounding environment unchanged.

Validation

Locally reproduced the false fallback under LC_ALL=de_DE.UTF-8 with a small derivation graph, then verified the patched mod_post no longer emits \dontUseTooLargeCoords and the summary embeds the DG correctly under both de_DE.UTF-8 and C.UTF-8.