feat(missions): JstPeriod helper — 02:00 JST anchored day/week/month keys

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

SVSim.EmulatedEntrypoint/Services/JstPeriod.cs

@@ -0,0 +1,53 @@
+using System.Globalization;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// JST-anchored period bucketing for ViewerEventCounters. Day/week/month boundaries are
+/// 02:00 JST (matching the real-game daily reset). Pure functions, no dependencies.
+/// </summary>
+public static class JstPeriod
+{
+    private static readonly TimeSpan Jst = TimeSpan.FromHours(9);
+    private const string DayPrefix   = "day:";
+    private const string WeekPrefix  = "week:";
+    private const string MonthPrefix = "month:";
+    public const string AllTime = "all-time";
+
+    /// <summary>
+    /// Converts the given instant to a JST-anchored "effective date" by:
+    /// 1. Shifting to JST (+09:00)
+    /// 2. Subtracting 2 hours so anything before 02:00 JST belongs to the previous day
+    /// </summary>
+    private static DateTime EffectiveJstDate(DateTimeOffset utcOrAny)
+    {
+        var jst = utcOrAny.ToOffset(Jst);
+        return jst.AddHours(-2).Date;
+    }
+
+    public static string DayKey(DateTimeOffset when)
+    {
+        var d = EffectiveJstDate(when);
+        return $"{DayPrefix}{d:yyyy-MM-dd}";
+    }
+
+    public static string WeekKey(DateTimeOffset when)
+    {
+        var d = EffectiveJstDate(when);
+        var iso = ISOWeek.GetWeekOfYear(d);
+        var year = ISOWeek.GetYear(d);
+        return $"{WeekPrefix}{year:D4}-W{iso:D2}";
+    }
+
+    public static string MonthKey(DateTimeOffset when)
+    {
+        var d = EffectiveJstDate(when);
+        return $"{MonthPrefix}{d:yyyy-MM}";
+    }
+
+    /// <summary>Returns [day, week, month, all-time] keys for the given instant.</summary>
+    public static IReadOnlyList<string> AllPeriods(DateTimeOffset when) => new[]
+    {
+        DayKey(when), WeekKey(when), MonthKey(when), AllTime,
+    };
+}