feat(ranking): stub /ranking/* (6 endpoints)

Rankings menu opens. Period picker shows deterministic monthly schedule.
Every leaderboard returns { period, ranking: [] }.

Endpoints:
- /ranking/get_viewable_ranking_period_list
- /ranking/master_point_{rotation,unlimited}_info
- /ranking/rank_match_class_win_{rotation,unlimited}_info
- /ranking/two_pick_win_info

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

SVSim.EmulatedEntrypoint/Models/Dtos/Ranking/PeriodEntryDto.cs

@@ -0,0 +1,26 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Ranking;
+
+/// <summary>
+/// Base period-entry shape used by /ranking/get_viewable_ranking_period_list's
+/// rank_match[] and crossover_* arrays. Master-point and two-pick variants
+/// add fields — see <see cref="MasterPointPeriodEntryDto"/> and
+/// <see cref="TwoPickPeriodEntryDto"/>.
+/// </summary>
+[MessagePackObject]
+public class PeriodEntryDto
+{
+    [JsonPropertyName("id"), Key("id")]
+    public string Id { get; set; } = "0";
+
+    [JsonPropertyName("period_num"), Key("period_num")]
+    public string PeriodNum { get; set; } = "0";
+
+    [JsonPropertyName("begin_time"), Key("begin_time")]
+    public string BeginTime { get; set; } = "";
+
+    [JsonPropertyName("end_time"), Key("end_time")]
+    public string EndTime { get; set; } = "";
+}