feat(battle-xp): wire story first-clear /finish through IBattleXpService

- StoryConfig.ClassXpPerClear retired; equivalent knob moves to
  BattleXpConfig.StoryXpPerClear (falls back to XpPerWin when null).
- StoryService.FinishAsync now grants class XP via IBattleXpService with
  BattleXpMode.Story on the firstClear && isPlayShape branch. Persists
  post-grant Level/Exp to Viewer.Classes.
- Wire fields (get_class_experience, class_level) stringified as before.
- StoryConfig kept as an empty [ConfigSection] for future story-specific
  knobs. StoryServiceTests updated to reflect the new default path
  (XpPerWin=100 crosses classexp.csv L1=50 → L2 with 50 carry).

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

SVSim.Database/Models/Config/StoryConfig.cs

@@ -1,9 +1,13 @@
 namespace SVSim.Database.Models.Config;
 
+/// <summary>
+/// Story-family placeholder config section. Class XP per clear moved to
+/// <see cref="BattleXpConfig.StoryXpPerClear"/> (BattleXpMode.Story) as part of the
+/// unified per-mode XP surface. Kept as an empty section so future story-specific
+/// knobs (dialogue speed, auto-skip, etc.) have a home.
+/// </summary>
 [ConfigSection("Story")]
 public class StoryConfig
 {
-    public int ClassXpPerClear { get; set; } = 200;
-
     public static StoryConfig ShippedDefaults() => new();
 }