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/Client/UI/HudUi.cs

@@ -146,8 +146,6 @@ namespace ProjectM.Client
             return e;
         }
 
-        /// <summary>True when the theme is loaded and a sprite is present (callers choose icon vs text fallback).</summary>
-
 
         /// <summary>
         /// An input-prompt chip: the Synty key/button glyph (left untinted to keep its printed face) when present,
@@ -170,7 +168,5 @@ namespace ProjectM.Client
             return cap;
         }
 
-        /// <summary>Apply a uniform 9-slice (horizontal / vertical source-texture px) to a skinned element.</summary>
-
     }
 }