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).

2 files changed, 17 insertions, 4 deletions

diff --git a/lib/naming/lang.ml b/lib/naming/lang.ml
index 92c8a9a..2301513 100644
--- a/lib/naming/lang.ml
+++ b/lib/naming/lang.ml
@@ -10,6 +10,7 @@ type t = {
   celebration : table;
   weekday : table;
   month : table;
+  month_abbr : table;
   season : table;
   rank : table;
   colour : table;
@@ -54,13 +55,17 @@ let month t n =
   if n < 1 || n > 12 then string_of_int n
   else match lookup t (fun x -> x.month) (string_of_int n) with Some v -> v | None -> string_of_int n
 
+let month_abbr t n =
+  if n < 1 || n > 12 then string_of_int n
+  else match lookup t (fun x -> x.month_abbr) (string_of_int n) with Some v -> v | None -> string_of_int n
+
 let code t = t.code
 let fallback_code t = t.fallback_code
 
 let raw =
   { code = "raw"; fallback_code = None; celebration = empty_table; weekday = empty_table;
-    month = empty_table; season = empty_table; rank = empty_table; colour = empty_table;
-    term = empty_table; rite = empty_table; chain = None }
+    month = empty_table; month_abbr = empty_table; season = empty_table; rank = empty_table;
+    colour = empty_table; term = empty_table; rite = empty_table; chain = None }
 
 let with_fallback t base = { t with chain = Some base }
 
@@ -96,6 +101,7 @@ let of_string text =
                celebration = find "celebration";
                weekday = find "weekday";
                month = find "month";
+               month_abbr = find "month_abbr";
                season = find "season";
                rank = find "rank";
                colour = find "colour";
@@ -107,6 +113,6 @@ let keys t =
   let qualify prefix m = SM.bindings m |> List.map (fun (k, v) -> (prefix ^ "." ^ k, v)) in
   List.concat
     [ qualify "celebration" t.celebration; qualify "weekday" t.weekday;
-      qualify "month" t.month; qualify "season" t.season; qualify "rank" t.rank;
-      qualify "colour" t.colour; qualify "term" t.term; qualify "rite" t.rite ]
+      qualify "month" t.month; qualify "month_abbr" t.month_abbr; qualify "season" t.season;
+      qualify "rank" t.rank; qualify "colour" t.colour; qualify "term" t.term; qualify "rite" t.rite ]
   |> List.sort compare
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