feat(templates): wall calendar grid in LaTeX, groff and HTML

Three flavours, not six: a month grid in Markdown or plain text is a
worse artefact than the booklet already is, and shipping a template we
would not use ourselves is maintenance with no reader.

These are the first templates to use weeks and in_month, so this is
where the view model earns its keep -- the booklet and the grid come
from one model with no second code path.

Every cell carries a per-cell 'last' boolean (already in the view). A
table row needs a separator BETWEEN cells and the engine deliberately
has no 'unless last' construct; the rule is shape the data, not the
template. Without it the LaTeX grid emits eight columns for seven
cells and pdflatex rejects the file.

Day cells resolve their label as {{#name}}{{la}}{{^la}}{{slug}}{{/la}}
{{/name}}, never a bare {{name.la}}: the enclosing month object has
its own name.la, and the engine's scope lookup falls back outward, so
a bare dotted lookup would render the month's own Latin name on every
day lacking one -- a wall calendar where every day reads "January".
Verified on the goldens: Ianuarius appears exactly once per file, the
month heading, never as a day label.

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