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/MonthlyRankingResponseDto.cs

@@ -0,0 +1,16 @@
+using System.Text.Json.Serialization;
+using MessagePack;
+
+namespace SVSim.EmulatedEntrypoint.Models.Dtos.Ranking;
+
+[MessagePackObject]
+public sealed class MonthlyRankingResponseDto
+{
+    [JsonPropertyName("period"), Key("period")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public PeriodEntryDto Period { get; set; } = new();
+
+    [JsonPropertyName("ranking"), Key("ranking")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.Never)]
+    public List<RankingEntryDto> Ranking { get; set; } = new();
+}