feat(ranking): add RankingPeriodSchedule helper

Pure deterministic monthly period generator for the four ranking
families. Anchor dates derived from prod capture (2026-06-09): id=1 is
each family's launch month in JST; id=N is anchor + N-1 months. Used
by /ranking/get_viewable_ranking_period_list to render the period
picker and by per-family leaderboard endpoints to echo the requested
period back.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.UnitTests/Services/RankingPeriodScheduleTests.cs

@@ -0,0 +1,79 @@
+using SVSim.EmulatedEntrypoint.Services;
+
+namespace SVSim.UnitTests.Services;
+
+public class RankingPeriodScheduleTests
+{
+    // 2026-06-09 17:00 UTC = 2026-06-10 02:00 JST — clearly in JST's June 2026 month.
+    private static readonly DateTime CaptureNowUtc =
+        new(2026, 6, 9, 17, 0, 0, DateTimeKind.Utc);
+
+    [Test]
+    public void RankMatch_at_capture_time_has_current_period_id_122()
+    {
+        var list = RankingPeriodSchedule.GenerateFor(
+            RankingPeriodSchedule.Family.RankMatch, CaptureNowUtc);
+
+        Assert.That(list, Is.Not.Empty);
+        Assert.That(list[0].Id, Is.EqualTo("122"));
+        Assert.That(list[0].BeginTime, Is.EqualTo("2026-06-01 02:00:00"));
+        Assert.That(list[0].EndTime, Is.EqualTo("2026-07-01 01:59:59"));
+        Assert.That(list.Count, Is.EqualTo(122));
+    }
+
+    [Test]
+    public void MasterPoint_at_capture_time_has_current_period_id_120()
+    {
+        var list = RankingPeriodSchedule.GenerateFor(
+            RankingPeriodSchedule.Family.MasterPoint, CaptureNowUtc);
+        Assert.That(list[0].Id, Is.EqualTo("120"));
+        Assert.That(list.Count, Is.EqualTo(120));
+    }
+
+    [Test]
+    public void TwoPick_at_capture_time_has_current_period_id_119()
+    {
+        var list = RankingPeriodSchedule.GenerateFor(
+            RankingPeriodSchedule.Family.TwoPick, CaptureNowUtc);
+        Assert.That(list[0].Id, Is.EqualTo("119"));
+        Assert.That(list.Count, Is.EqualTo(119));
+    }
+
+    [Test]
+    public void Sealed_at_capture_time_has_current_period_id_62()
+    {
+        var list = RankingPeriodSchedule.GenerateFor(
+            RankingPeriodSchedule.Family.Sealed, CaptureNowUtc);
+        Assert.That(list[0].Id, Is.EqualTo("62"));
+        Assert.That(list.Count, Is.EqualTo(62));
+    }
+
+    [Test]
+    public void Schedule_is_descending_by_id()
+    {
+        var list = RankingPeriodSchedule.GenerateFor(
+            RankingPeriodSchedule.Family.RankMatch, CaptureNowUtc);
+        for (int i = 1; i < list.Count; i++)
+        {
+            Assert.That(int.Parse(list[i - 1].Id), Is.GreaterThan(int.Parse(list[i].Id)),
+                $"position {i - 1} ({list[i - 1].Id}) should be > position {i} ({list[i].Id})");
+        }
+    }
+
+    [Test]
+    public void TryFindById_returns_entry_when_id_in_range()
+    {
+        var entry = RankingPeriodSchedule.TryFindById(
+            RankingPeriodSchedule.Family.RankMatch, periodId: 122, CaptureNowUtc);
+        Assert.That(entry, Is.Not.Null);
+        Assert.That(entry!.BeginTime, Is.EqualTo("2026-06-01 02:00:00"));
+    }
+
+    [Test]
+    public void TryFindById_returns_null_for_unknown_id()
+    {
+        var entry = RankingPeriodSchedule.TryFindById(
+            RankingPeriodSchedule.Family.RankMatch, periodId: 9999, CaptureNowUtc);
+        Assert.That(entry, Is.Null);
+    }
+}