refactor(battlenode): rename mode-id field off BattleType, add BattleModes (§D)

Behavior-preserving; 271 BattleNode/Matching/Services tests green, full solution builds.

"BattleType" meant two things: the Sessions.BattleType enum (Pvp/Bot) and an int
"mode id" field. Renamed the int field on MatchContext AND the BattleStartBody wire
DTO to BattleModeId (wire key stays "battleType" via JsonPropertyName), so BattleType
now means only the enum project-wide.

New Bridge/BattleModes.cs (TakeTwo = 11) replaces every 11 literal — both prod
MatchContextBuilder sites and the test fixtures/assertions. The arbitrary-passthrough
42 and bot 0 stay literal.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Bridge/MatchContext.cs

@@ -25,5 +25,8 @@ public sealed record MatchContext(
     int    FieldId,
     int    IsOfficial,      // 0 or 1
 
-    // Battle-mode hint, currently TK2 == 11. Future modes populate their own value.
-    int    BattleType);
+    // Battle-mode hint (the prod do_matching mode id). Named BattleModeId, NOT BattleType, to
+    // avoid colliding with the <see cref="Sessions.BattleType"/> enum (Pvp/Bot) — a different axis.
+    // Known values live in <see cref="BattleModes"/> (currently just TK2 == 11). Future modes add
+    // their own constant.
+    int    BattleModeId);