feat(render): Roman week numbers and date spans, as data

The ordo booklet's week header repeated the month name on every week
even though the heading above already established it -- pure noise.
Replace it with a Roman week number plus the span of dates the week
covers, e.g. "Hebdomada I (Ian 1-2)", following the project's own rule
that a presentation choice is data, not code.

lang/{la,en}.ini gain a [month_abbr] section (three-letter month
abbreviations); Lang.month_abbr follows Lang.month's exact shape,
including the out-of-range and miss-returns-the-key contracts. The
coverage test now fails loudly if an abbreviation goes missing, the
same as [month] already does.

Every week object in the view gains num_roman (Roman numeral, num
stays as the arabic original -- Roman is a presentation choice, not an
engine change), first_dom/last_dom (the day-of-month of the week's
first and last IN-MONTH days, padding excluded), month_abbr (resolved
through Lang.month_abbr), and single_day (true when the week holds
exactly one in-month day). single_day is a flag, not a preformatted
span string: the engine is logic-less and cannot itself decide between
"Ian 1" and "Ian 1-2", so a template makes that call from the flag
instead -- the same "shape the data, not the template" discipline
in_month and last already follow.

Weeks are built per month with padding only at the two ends, so a
week's in-month days never cross a month boundary -- verified, not
assumed: every week always has at least one real day since no month is
shorter than a single week.

Covered by three new View tests, including a real single-day-week
witness (January 2027's own trailing week is a lone Sunday, the 31st).

1 files changed, 19 insertions, 0 deletions

diff --git a/lang/la.ini b/lang/la.ini
index 9cbb8fe..536f717 100644
--- a/lang/la.ini
+++ b/lang/la.ini
@@ -66,6 +66,25 @@ saturday  = Sabbatum
 11 = November
 12 = December
 
+[month_abbr]
+; Three-letter abbreviations for the printed booklet's week header (a
+; span like "Ian 1-2" beside a Roman week number, so the header need not
+; repeat the month name spelled out in full). Each is simply the first
+; three letters of [month]'s own name above -- no separate sourcing, since
+; this is general vocabulary rather than a Missal heading either.
+1  = Ian
+2  = Feb
+3  = Mar
+4  = Apr
+5  = Mai
+6  = Iun
+7  = Iul
+8  = Aug
+9  = Sep
+10 = Oct
+11 = Nov
+12 = Dec
+
 [season]
 ; RG 71-77's own season names. Five of eight are direct Proprium de Tempore
 ; section headings in docs/research/LT.txt (line numbers below); the other