kernel+ef: resolve readings, chain steps 1 and 2

Liturgical_day.citations has read "always empty until Plan 4" since Plan 3;
it is now filled. Rite.t gains a readings function, rite-supplied for the
same reason transfer_target is: what a day with no proper falls back to is a
rubric, not a universal. Calendar calls it and passes its own temporal
function as the callback the rite needs to reach another date.

Steps 1 and 2 only: the observed celebration's own proper, else the day's own
temporal slug. Nothing encodes "Lent has daily propers" -- the presence of an
entry is the discriminator.

3 files changed, 23 insertions, 1 deletions

diff --git a/lib/kernel/calendar.ml b/lib/kernel/calendar.ml
index cffdf69..724641d 100644
--- a/lib/kernel/calendar.ml
+++ b/lib/kernel/calendar.ml
@@ -435,7 +435,9 @@ let build_day (rite : ('s, 'r) Rite.t) (idx : 'r Layer.by_date)
     transferred_in;
     transferred_out;
     omitted;
-    citations = [];
+    citations =
+      rite.Rite.readings ~observed:resolution.Precedence.observed.Precedence.cel ~temporal ~date
+        ~temporal_at:rite.Rite.temporal;
   }
 
 let year (rite : ('s, 'r) Rite.t) (layer : 'r Layer.t) (y : int) :
diff --git a/lib/kernel/rite.ml b/lib/kernel/rite.ml
index 89fceb6..91326a6 100644
--- a/lib/kernel/rite.ml
+++ b/lib/kernel/rite.ml
@@ -11,4 +11,10 @@ type ('s, 'r) t = {
   season_runs : 's list;
   transfer_target :
     'r Precedence.candidate -> Date.t -> (Date.t -> 'r Celebration.t) -> Date.t;
+  readings :
+    observed:'r Celebration.t ->
+    temporal:('s, 'r) Temporal.t ->
+    date:Date.t ->
+    temporal_at:(Date.t -> ('s, 'r) Temporal.t) ->
+    Citation.t list;
 }
diff --git a/lib/kernel/rite.mli b/lib/kernel/rite.mli
index ffe9471..a41e034 100644
--- a/lib/kernel/rite.mli
+++ b/lib/kernel/rite.mli
@@ -59,4 +59,18 @@ type ('s, 'r) t = {
           [transfer_target] for a concrete termination argument (a
           structural step bound, not an appeal to the real calendar's own
           structure). *)
+  readings :
+    observed:'r Celebration.t ->
+    temporal:('s, 'r) Temporal.t ->
+    date:Date.t ->
+    temporal_at:(Date.t -> ('s, 'r) Temporal.t) ->
+    Citation.t list;
+      (** The day's Epistle and Gospel citations, or []. Rite-supplied for the
+          same reason [transfer_target] is: what a day with no proper of its
+          own falls back to is a rubric of a particular rite, not a universal.
+
+          [temporal_at] is a callback so the rite can reach another date's
+          temporal identity (the preceding Sunday's, for the ferial rule)
+          without re-implementing the temporal cycle -- the same shape
+          [transfer_target]'s own [occupant] callback established. *)
 }