feat: support safe recurrence additions

Parse, persist, expand, and round-trip compatible RDATE values while preserving whole-series TUI behavior and conservative rewrite guards.

Verified: normal, warning-as-error, and ASan/UBSan builds pass all 9 suites; storage failure paths preserve calendar and backup bytes; live PTY restores terminal state.

tests/calendar_transfer_tests.cpp

@@ -65,6 +65,7 @@ void check_invalid_argument(Function&& function, const std::string_view message)
     recurrence.by_month_day = {1, -1};
     recurrence.by_month = {1, 7};
     event.recurrence = std::move(recurrence);
+    event.recurrence_additions = {at(20)};
     event.recurrence_exceptions = {at(16), at(18)};
     return event;
 }
@@ -79,6 +80,7 @@ void check_invalid_argument(Function&& function, const std::string_view message)
            left.time_basis == right.time_basis &&
            left.time_zone == right.time_zone &&
            left.recurrence == right.recurrence &&
+           left.recurrence_additions == right.recurrence_additions &&
            left.recurrence_exceptions == right.recurrence_exceptions;
 }
 
@@ -163,6 +165,8 @@ void test_every_event_field_participates_in_collision_equality()
         {"time basis", [](auto& event) { event.time_basis = nocal::TimeBasis::utc; }},
         {"time zone", [](auto& event) { event.time_zone = "America/New_York"; }},
         {"recurrence", [](auto& event) { event.recurrence->interval += 1; }},
+        {"recurrence additions",
+         [](auto& event) { event.recurrence_additions.push_back(at(19)); }},
         {"recurrence exclusions",
          [](auto& event) { event.recurrence_exceptions.push_back(at(19)); }},
     };