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, 7 insertions, 0 deletions

diff --git a/lib/naming/lang.mli b/lib/naming/lang.mli
index 4d7f82f..34a6d5d 100644
--- a/lib/naming/lang.mli
+++ b/lib/naming/lang.mli
@@ -61,6 +61,13 @@ val weekday : t -> int -> string
 (** [month t n], 1 = January. Out-of-range [n] returns [string_of_int n]. *)
 val month : t -> int -> string
 
+(** [month_abbr t n], 1 = January, e.g. ["Ian"]/["Jan"]. Same shape as
+    {!month}: out-of-range [n] returns [string_of_int n], and a miss falls
+    back through the chain to [string_of_int n] as well, never to [month]'s
+    own full name -- an abbreviation is its own vocabulary, not a truncation
+    of another lookup. *)
+val month_abbr : t -> int -> string
+
 (** Every (section-qualified key, value) pair, sorted. Used by [lang --dump] and
     [lang --check]. Keys are qualified as e.g. ["celebration.ef-epiphany"]. *)
 val keys : t -> (string * string) list