feat(render): iCalendar emitter, RFC 5545

Not a template job: folding, escaping, exclusive DTEND and stable UIDs
are rules a logic-less template cannot enforce, and each fails silently
in a subscriber's client rather than loudly at generation.

  DTEND is EXCLUSIVE for an all-day event (section 3.6.1). Wrong here
  shows every event a day short, everywhere.
  UIDs are YYYYMMDD-<rite>@colitur and stable across regenerations
  (section 3.8.4.7). Wrong here duplicates the whole year in every
  subscriber's phone, months later.
  Every line is CRLF-terminated and folded at 75 octets (section 3.1).
  No RRULE: a liturgical calendar is not a recurrence rule. Asserted, so
  nobody optimises it later.

DTSTAMP is a parameter, not a clock read. RFC 5545 requires it and the
obvious implementation reads the wall clock -- which violates the
kernel's determinism rule and would make two feeds from identical data
differ byte-for-byte, defeating reproducible builds and any reviewable
diff on a published tree.

Corrected one test literal against real engine output: DTSTAMP is a
per-VEVENT property (section 3.8.7.2), not calendar-level, so the
default-value line count is 365 (every event), not 1.

Mutation-tested: a non-exclusive DTEND reddens the suite.

2 files changed, 82 insertions, 0 deletions

diff --git a/lib/render/emit_ics.ml b/lib/render/emit_ics.ml
new file mode 100644
index 0000000..e1a5ad0
--- /dev/null
+++ b/lib/render/emit_ics.ml
@@ -0,0 +1,67 @@
+(* emit_ics.ml *)
+module T = Template
+
+let get v k = match v with T.Obj kvs -> List.assoc_opt k kvs | _ -> None
+let s v k = match get v k with Some (T.Str x) -> x | _ -> ""
+
+let compact iso = (* "2027-01-13" -> "20270113" *)
+  String.concat "" (String.split_on_char '-' iso)
+
+(* Civil-date successor without touching the kernel: parse, add, re-format via
+   Colitur_kernel.Date, which is already total and validated over 1583..9999. *)
+let next_day iso =
+  match Colitur_kernel.Date.of_iso8601 iso with
+  | Ok d -> Colitur_kernel.Date.to_iso8601 (Colitur_kernel.Date.add_days d 1)
+  | Error _ -> iso
+
+let esc x = Escape.apply Escape.Ics x
+let line b l = Buffer.add_string b (Escape.fold_ics l)
+
+let event b ~rite ~dtstamp d =
+  let iso = s d "iso" in
+  if iso <> "" then begin
+    let name =
+      match get d "name" with
+      | Some (T.Obj kvs) -> (
+          match List.assoc_opt "en" kvs with
+          | Some (T.Str x) when x <> "" -> x
+          | _ -> ( match List.assoc_opt "la" kvs with Some (T.Str x) -> x | _ -> s d "slug"))
+      | _ -> s d "slug"
+    in
+    let summary = Printf.sprintf "%s (%s, %s)" name (s d "rank") (s d "colour") in
+    let desc =
+      String.concat "\n"
+        (List.filter (fun x -> x <> "")
+           [ (if s d "first" <> "" then "Epistle " ^ s d "first" else "");
+             (if s d "gospel" <> "" then "Gospel " ^ s d "gospel" else "") ])
+    in
+    line b "BEGIN:VEVENT";
+    (* RFC 5545 section 3.8.4.7: the UID must be stable across regenerations,
+       or every subscriber gets a duplicate of the whole year. *)
+    line b (Printf.sprintf "UID:%s-%s@colitur" (compact iso) rite);
+    line b ("DTSTAMP:" ^ dtstamp);
+    line b ("DTSTART;VALUE=DATE:" ^ compact iso);
+    (* RFC 5545 section 3.6.1: DTEND is EXCLUSIVE for an all-day event. *)
+    line b ("DTEND;VALUE=DATE:" ^ compact (next_day iso));
+    line b ("SUMMARY:" ^ esc summary);
+    if desc <> "" then line b ("DESCRIPTION:" ^ esc desc);
+    line b "TRANSP:TRANSPARENT";
+    line b "END:VEVENT"
+  end
+
+let year ?dtstamp ?calname v =
+  let rite = s v "rite" in
+  let y = s v "year" in
+  let dtstamp = match dtstamp with Some x -> x | None -> y ^ "0101T000000Z" in
+  let calname = match calname with Some x -> x | None -> "colitur " ^ rite ^ " " ^ y in
+  let b = Buffer.create (256 * 1024) in
+  line b "BEGIN:VCALENDAR";
+  line b "VERSION:2.0";
+  line b "PRODID:-//colitur//liturgical calendar//EN";
+  line b "CALSCALE:GREGORIAN";
+  line b "METHOD:PUBLISH";
+  (* Non-standard but universally honoured; without it clients show the URL. *)
+  line b ("X-WR-CALNAME:" ^ esc calname);
+  (match get v "days" with Some (T.List l) -> List.iter (event b ~rite ~dtstamp) l | _ -> ());
+  line b "END:VCALENDAR";
+  Buffer.contents b
diff --git a/lib/render/emit_ics.mli b/lib/render/emit_ics.mli
new file mode 100644
index 0000000..270bd76
--- /dev/null
+++ b/lib/render/emit_ics.mli
@@ -0,0 +1,15 @@
+(* emit_ics.mli *)
+(** RFC 5545 iCalendar. NOT a template job: folding, escaping, exclusive DTEND
+    and stable UIDs are rules a logic-less template cannot enforce, and getting
+    any of them wrong produces a feed that fails silently in a subscriber's
+    client (spec section 6). *)
+
+(** [year ?dtstamp ?calname view].
+
+    [dtstamp] defaults to [YYYY0101T000000Z] for the view's own year. It is a
+    PARAMETER, never a clock read: RFC 5545 requires DTSTAMP, the obvious
+    implementation reads the wall clock, and that would both violate the
+    kernel's determinism rule and make two feeds from identical data differ
+    byte-for-byte -- defeating reproducible builds and a reviewable git diff on
+    a published tree. Same data in, same bytes out. *)
+val year : ?dtstamp:string -> ?calname:string -> Template.value -> string