feat(render): the view model

Shapes a civil year of resolved days into the value a template renders
against. This layer is why the engine can stay logic-less: a month grid
needs leading blank cells, week bucketing and an in-month test, and a
logic-less template can compute none of it.

Both weeks and days are offered at every level -- the booklet walks
days, the grid walks weeks -- so the two artefacts cannot drift.

Colours are six booleans, not hex: hex bakes a presentation policy into
the engine, and LaTeX, groff and HTML each want a different colour
expression. Asserted: exactly one of the six is true on every day of a
whole year, so a template keying off them can never get none or two.

Padding cells carry every field a real day carries, empty, so a template
never hits a missing key mid-grid.

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+(** Shapes a civil year of resolved days into the value a template renders
+    against (spec section 4).
+
+    This layer exists because a month grid needs leading blank cells, week
+    bucketing and an "is this cell in the current month" test, and a logic-less
+    template can compute none of it. Shaping the data here keeps the engine
+    logic-less AND makes the grid template trivial. The view model IS the design.
+
+    Both [weeks] and [days] are offered at every level: the booklet walks
+    [days], the grid walks [weeks]. One model, two artefacts, no second code
+    path that could drift.
+
+    Colours are exposed as six booleans rather than a hex string: hex would bake
+    a presentation policy into the engine, and LaTeX, groff and HTML each want a
+    different colour expression. Exactly one of the six is true on every day. *)
+
+val of_days :
+  vocab:('s, 'r) Colitur_kernel.Vocab.t ->
+  rite:string ->
+  year:int ->
+  ('s, 'r) Colitur_kernel.Liturgical_day.t list ->
+  Template.value
+(** [of_days ~vocab ~rite ~year days] where [days] is one civil year, 1 January
+    to 31 December, in order. Pure and total. *)