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.EmulatedEntrypoint/Services/RankingPeriodSchedule.cs

@@ -0,0 +1,95 @@
+using System.Globalization;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// Pure deterministic monthly period schedule for /ranking/*. Each ranking family
+/// (RankMatch, MasterPoint, TwoPick, Sealed) launched in a different month on the
+/// live server; id=1 in each family is its launch month. id=current means "this
+/// month in JST." The generator returns descending-by-id (newest first).
+///
+/// Anchor dates derived from prod capture 2026-06-09 17:00 UTC:
+///   RankMatch current id = 122 → launch month = 2026-06 minus 121 months = 2016-05
+///   MasterPoint current id = 120 → 2016-07
+///   TwoPick current id = 119 → 2016-08
+///   Sealed current id = 62 → 2021-05
+///
+/// See docs/superpowers/specs/2026-06-10-ranking-stubs-design.md for rationale.
+/// </summary>
+public static class RankingPeriodSchedule
+{
+    public enum Family { RankMatch, MasterPoint, TwoPick, Sealed }
+
+    // (Year, Month) of each family's id=1 month, JST.
+    private static readonly Dictionary<Family, (int Year, int Month)> FamilyAnchors = new()
+    {
+        [Family.RankMatch]   = (2016, 5),
+        [Family.MasterPoint] = (2016, 7),
+        [Family.TwoPick]     = (2016, 8),
+        [Family.Sealed]      = (2021, 5),
+    };
+
+    private static readonly TimeZoneInfo Jst = TimeZoneInfo.FindSystemTimeZoneById("Tokyo Standard Time");
+
+    public static IReadOnlyList<PeriodEntry> GenerateFor(Family family, DateTime nowUtc)
+    {
+        var nowJst = TimeZoneInfo.ConvertTimeFromUtc(nowUtc, Jst);
+        var anchor = FamilyAnchors[family];
+        int currentId = MonthsBetweenInclusive(anchor.Year, anchor.Month, nowJst.Year, nowJst.Month);
+        if (currentId < 1) return Array.Empty<PeriodEntry>();
+
+        var result = new List<PeriodEntry>(currentId);
+        for (int id = currentId; id >= 1; id--)
+        {
+            result.Add(BuildEntry(family, id, anchor));
+        }
+        return result;
+    }
+
+    public static PeriodEntry? TryFindById(Family family, int periodId, DateTime nowUtc)
+    {
+        if (periodId < 1) return null;
+        var nowJst = TimeZoneInfo.ConvertTimeFromUtc(nowUtc, Jst);
+        var anchor = FamilyAnchors[family];
+        int currentId = MonthsBetweenInclusive(anchor.Year, anchor.Month, nowJst.Year, nowJst.Month);
+        if (periodId > currentId) return null;
+        return BuildEntry(family, periodId, anchor);
+    }
+
+    private static PeriodEntry BuildEntry(Family family, int id, (int Year, int Month) anchor)
+    {
+        // id=1 is the anchor month; id=N is anchor month + (N-1) months.
+        int totalMonths = (anchor.Year * 12 + (anchor.Month - 1)) + (id - 1);
+        int year = totalMonths / 12;
+        int month = (totalMonths % 12) + 1;
+
+        var begin = new DateTime(year, month, 1, 2, 0, 0);
+        // End = first day of next month at 02:00:00 minus 1 second = "YYYY-MM+1-01 01:59:59"
+        var end = begin.AddMonths(1).AddSeconds(-1);
+
+        int periodNum = family switch
+        {
+            // Captured offsets: RankMatch period_num = id - 1; MasterPoint period_num = id - 1.
+            // (Capture frame 64 shows rank_match[0] = { id:122, period_num:121 } and
+            //  master_point[0] = { id:120, period_num:119 }, two_pick[0] = { id:119, period_num:119 }.)
+            Family.RankMatch   => id - 1,
+            Family.MasterPoint => id - 1,
+            _                  => id,
+        };
+
+        return new PeriodEntry(
+            Id: id.ToString(CultureInfo.InvariantCulture),
+            PeriodNum: periodNum.ToString(CultureInfo.InvariantCulture),
+            BeginTime: begin.ToString("yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture),
+            EndTime: end.ToString("yyyy-MM-dd HH:mm:ss", CultureInfo.InvariantCulture));
+    }
+
+    private static int MonthsBetweenInclusive(int startYear, int startMonth, int endYear, int endMonth)
+        => (endYear - startYear) * 12 + (endMonth - startMonth) + 1;
+}
+
+public sealed record PeriodEntry(
+    string Id,
+    string PeriodNum,
+    string BeginTime,
+    string EndTime);