fix(render): omit DTEND at the domain's own last day, 9999-12-31

F1: Date.add_days is UNBOUNDED (date.mli) -- only Date.make enforces
1583..9999 -- and Date.to_iso8601 pads but never truncates, so
9999-12-31's naive successor formatted as "10000-01-01", and compact
turned that into a 9-digit, non-conformant DATE on the last VEVENT of
year 9999. Confirmed at the source before fixing, and reproduced
against real `colitur emit --format ics --from 9999 --to 9999` output
(DTEND;VALUE=DATE:100000101) before touching any code.

RFC 5545 section 3.6.1: a VEVENT with a DATE-valued DTSTART and
neither DTEND nor DURATION has an implicit one-day duration, so
omitting DTEND for that one event is the standard's own correct
answer, not a workaround. dtend_of re-derives the successor's
year/month/day and re-validates them through Date.make -- the one
function that actually enforces the domain -- before trusting the
string; None means the caller omits the DTEND line entirely.

F2 (minor, same function): documented next_day's own Error branch as
dead-but-silent on shipped data (event's iso <> "" guard is the only
caller and always parses) -- behaviour unchanged, comment only.

Two new tests: the domain's last VEVENT (DTSTART 99991231) has no
DTEND line at all; every DTEND anywhere in a 9999 feed is exactly 8
digits (the general form of the bug, catches a regression anywhere
else in the domain too). Existing 2027/2028 DTEND-arithmetic
assertions untouched and still pass.

Mutation-proved: both new tests fail against the pre-fix code
(9-digit DTEND value caught verbatim), pass after.

1 files changed, 44 insertions, 1 deletions

diff --git a/test/test_ics.ml b/test/test_ics.ml
index ac0c9ee..7d1ca37 100644
--- a/test/test_ics.ml
+++ b/test/test_ics.ml
@@ -88,6 +88,47 @@ let test_dtstamp_is_a_parameter () =
   Alcotest.(check int) "default is deterministic" 365
     (count_prefix "DTSTAMP:20270101T000000Z" (lines (out_2027 ())))
 
+(* F1 (fix round 1, 2026-08-19): 9999 is the domain's own last civil year.
+   Its last VEVENT's successor date (10000-01-01) falls outside the
+   1583..9999 domain [Date.make] enforces (date.mli) -- [Date.add_days] is
+   itself unbounded, and [to_iso8601] pads but never truncates, so the naive
+   successor string would be 9 digits, not 8. RFC 5545 section 3.6.1 makes a
+   DATE-valued DTSTART with neither DTEND nor DURATION mean exactly one day,
+   so [dtend_of] omits the DTEND line entirely for that one event rather than
+   emit it malformed. *)
+let out_9999 () = Ics.year (Test_view.view_of 9999)
+
+let test_dtend_omitted_at_domain_end () =
+  let ls = Array.of_list (lines (out_9999 ())) in
+  let n = Array.length ls in
+  let idx = ref (-1) in
+  Array.iteri (fun i l -> if l = "DTSTART;VALUE=DATE:99991231" then idx := i) ls;
+  if !idx < 0 then Alcotest.fail "DTSTART;VALUE=DATE:99991231 not found";
+  let next_line = if !idx + 1 < n then ls.(!idx + 1) else "" in
+  Alcotest.(check bool) "no DTEND line follows DTSTART 99991231" true
+    (count_prefix "DTEND;VALUE=DATE:" [ next_line ] = 0);
+  Alcotest.(check bool) "SUMMARY follows DTSTART directly instead" true
+    (count_prefix "SUMMARY:" [ next_line ] = 1)
+
+(* The general form of F1's bug class: every DTEND value anywhere in the
+   feed must be an 8-digit YYYYMMDD, never 9 (a future domain-boundary
+   regression anywhere else in 1583..9999 would show up here too). *)
+let test_all_9999_dtends_are_eight_digits () =
+  let ls = lines (out_9999 ()) in
+  let prefix = "DTEND;VALUE=DATE:" in
+  let ends =
+    List.filter_map
+      (fun l ->
+        if count_prefix prefix [ l ] = 1 then
+          Some (String.sub l (String.length prefix) (String.length l - String.length prefix))
+        else None)
+      ls
+  in
+  Alcotest.(check bool) "at least one DTEND present in 9999" true (List.length ends > 0);
+  List.iter
+    (fun v -> if String.length v <> 8 then Alcotest.failf "DTEND value %S is %d digits, not 8" v (String.length v))
+    ends
+
 let suite =
   ( "Emit/ics",
     [ Alcotest.test_case "envelope" `Quick test_envelope;
@@ -97,4 +138,6 @@ let suite =
       Alcotest.test_case "folded and CRLF" `Quick test_every_line_folded_and_crlf;
       Alcotest.test_case "no RRULE" `Quick test_no_rrule;
       Alcotest.test_case "text escaped" `Quick test_text_escaped;
-      Alcotest.test_case "DTSTAMP is a parameter" `Quick test_dtstamp_is_a_parameter ] )
+      Alcotest.test_case "DTSTAMP is a parameter" `Quick test_dtstamp_is_a_parameter;
+      Alcotest.test_case "DTEND omitted at domain end" `Quick test_dtend_omitted_at_domain_end;
+      Alcotest.test_case "all 9999 DTENDs are eight digits" `Quick test_all_9999_dtends_are_eight_digits ] )