Hygiene B2: dead-code removal

Delete (unreferenced in any scene, per operator decision):
- TrainingDummy* chain (authoring/spawner/system/components/prefab) + UpgradePickup* chain (authoring/spawner/systems/components/prefab); remove TrainingDummyTag from the 3 live combat queries (AbilityFire/Melee/HealthApplyDamage now key on EnemyTag) and repoint HealthApplyDamageSystemTests to EnemyTag.
- Dead RPC RegionTransitSystem + RegionTransitRequest (no sender, ungated) + its test.
- Heartbeat + HeartbeatSystem (no WorldSystemFilter, ran no-op every tick) + its test.
- BuildSendSystem dead conveyor/pylon dev hooks (s_ConveyorDir, [/] rotation, Place{Pylon,Harvester,Conveyor} statics).
- Deprecated CyclePhase alias consts (Expedition/Defend/Build/*Ticks); repoint WaveSystemTests to Siege/Calm.
- Unread FeelConfig fields (HitFlashDurationMs, RumbleHeavy) + dangling HudUi doc-comments + stale crefs.

451/451 EditMode tests pass. CLAUDE.md HeartbeatSystemTests example ref to be repointed in B7.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Assets/_Project/Scripts/Server/Combat/HealthApplyDamageSystem.cs

@@ -13,7 +13,7 @@ namespace ProjectM.Server
     /// this tick), subtracts the summed amount from <see cref="Health"/>, then clears the buffer so
     /// each hit is applied exactly once. Entities that carry character stats (players) clamp to their
     /// data-driven <see cref="EffectiveCharacterStats.MaxHealth"/> ceiling; others (training dummies)
-    /// clamp at zero. A dead <see cref="TrainingDummyTag"/> is destroyed; player death is deferred.
+    /// clamp at zero. A dead <see cref="EnemyTag"/> is destroyed; player death is deferred.
     /// Health.Current is a <c>[GhostField]</c>, so the new value replicates to clients for display.
     ///
     /// Runs server-only (<see cref="WorldSystemFilterFlags.ServerSimulation"/>) inside the prediction
@@ -156,7 +156,7 @@ namespace ProjectM.Server
                             if (SystemAPI.HasComponent<IsLunging>(entity)) SystemAPI.SetComponentEnabled<IsLunging>(entity, false);
                         }
                     }
-                    else if (SystemAPI.HasComponent<TrainingDummyTag>(entity) || SystemAPI.HasComponent<EnemyTag>(entity) || SystemAPI.HasComponent<Destructible>(entity))
+                    else if (SystemAPI.HasComponent<EnemyTag>(entity) || SystemAPI.HasComponent<Destructible>(entity))
                         ecb.DestroyEntity(entity);
                 }
             }