Implement weekly and custom day-set recurrence (#27)

tests/test_recurrence_daily.py

@@ -214,13 +214,14 @@ def test_occurrences_between_year_boundary() -> None:
 
 
 def test_occurs_on_rejects_unimplemented_shapes_loudly() -> None:
-    """Weekly and beyond land with their own engine tasks; a loud
+    """Monthly/yearly land with their own engine tasks; a loud
     NotImplementedError beats a silent wrong answer."""
     rule = ScheduleRule(
-        rule_type=RuleType.WEEKLY, weekday_set={0}, start_date="2026-09-01"
+        rule_type=RuleType.MONTHLY_DAY, day_of_month=15,
+        start_date="2026-09-01",
     )
     with pytest.raises(NotImplementedError):
-        occurs_on(rule, _d("2026-09-07"))
+        occurs_on(rule, _d("2026-09-15"))
 
 def test_interval_7_preserves_weekday_alignment_across_months() -> None:
     """Weekly-by-interval semantics: every firing lands on the SAME
@@ -272,7 +273,7 @@ def test_occurs_on_unimplemented_shape_raises_even_outside_window() -> None:
     """An unimplemented shape must raise whether the date is inside or
     outside the window — silent False would hide a missing engine."""
     rule = ScheduleRule(
-        rule_type=RuleType.WEEKLY, weekday_set={0},
+        rule_type=RuleType.MONTHLY_DAY, day_of_month=15,
         start_date="2026-09-01", end_date="2026-09-10",
     )
     # Before start, inside window, after end: all must raise.
@@ -285,7 +286,8 @@ def test_occurrences_between_unimplemented_shape_raises_even_disjoint(
     tmp_path,
 ) -> None:
     rule = ScheduleRule(
-        rule_type=RuleType.WEEKLY, weekday_set={0}, start_date="2026-09-01"
+        rule_type=RuleType.MONTHLY_DAY, day_of_month=15,
+        start_date="2026-09-01"
     )
     # A range entirely before the rule's start still raises: the shape
     # is unimplemented, and silence would hide it.