cli: colitur easter <year> prints Easter and its anchors

Argv-parsed subcommand; validates the 1583..9999 domain (exit 2 otherwise);
prints 'name YYYY-MM-DD' lines. Cram test covers the output and the exit codes.

4 files changed, 53 insertions, 2 deletions

diff --git a/bin/main.ml b/bin/main.ml
index 761f68e..eb70a26 100644
--- a/bin/main.ml
+++ b/bin/main.ml
@@ -1,2 +1,28 @@
-(* colitur CLI — scaffold stub; the real `easter` subcommand arrives in Task 4. *)
-let () = ()
+module D = Colitur_kernel.Date
+module C = Colitur_kernel.Computus
+
+let fmt d = Printf.sprintf "%04d-%02d-%02d" (D.year d) (D.month d) (D.day d)
+
+let easter_report y =
+  [ ("easter", C.gregorian_easter y);
+    ("ash-wednesday", C.ash_wednesday y);
+    ("palm-sunday", C.palm_sunday y);
+    ("ascension", C.ascension y);
+    ("pentecost", C.pentecost y);
+    ("corpus-christi", C.corpus_christi y) ]
+  |> List.iter (fun (name, d) -> Printf.printf "%s %s\n" name (fmt d))
+
+let usage () =
+  prerr_endline "colitur: usage: colitur easter <year>";
+  exit 2
+
+let () =
+  match Sys.argv with
+  | [| _; "easter"; ys |] -> (
+      match int_of_string_opt ys with
+      | Some y when y >= 1583 && y <= 9999 -> easter_report y
+      | Some y ->
+          Printf.eprintf "colitur: year %d out of range 1583..9999\n" y;
+          exit 2
+      | None -> usage ())
+  | _ -> usage ()
diff --git a/dune-project b/dune-project
index 8a1fa58..cd3af07 100644
--- a/dune-project
+++ b/dune-project
@@ -1,6 +1,7 @@
 (lang dune 3.0)
 (name colitur)
 (generate_opam_files true)
+(cram enable)
 
 (package
  (name colitur)
diff --git a/test/cli.t b/test/cli.t
new file mode 100644
index 0000000..319fc8c
--- /dev/null
+++ b/test/cli.t
@@ -0,0 +1,21 @@
+Easter and its Easter-relative movable feasts for a year:
+
+  $ colitur easter 2026
+  easter 2026-04-05
+  ash-wednesday 2026-02-18
+  palm-sunday 2026-03-29
+  ascension 2026-05-14
+  pentecost 2026-05-24
+  corpus-christi 2026-06-04
+
+A year outside the supported domain is rejected (exit 2):
+
+  $ colitur easter 1000
+  colitur: year 1000 out of range 1583..9999
+  [2]
+
+No/garbage arguments give a usage error (exit 2):
+
+  $ colitur
+  colitur: usage: colitur easter <year>
+  [2]
diff --git a/test/dune b/test/dune
index 3d8df19..8ee8b87 100644
--- a/test/dune
+++ b/test/dune
@@ -1,3 +1,6 @@
 (test
  (name test_colitur)
  (libraries colitur_kernel alcotest qcheck qcheck-alcotest))
+
+(cram
+ (deps %{bin:colitur}))