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/Simulation/Combat/Destructible.cs

@@ -4,7 +4,7 @@ namespace ProjectM.Simulation
 {
     /// <summary>
     /// EB-1 — opt-in marker for an entity that <see cref="ProjectM.Server.HealthApplyDamageSystem"/> DESTROYS when
-    /// its <see cref="Health"/> hits 0 (alongside <c>TrainingDummyTag</c>/<c>EnemyTag</c>). Baked ONLY on the
+    /// its <see cref="Health"/> hits 0 (alongside <c>EnemyTag</c>). Baked ONLY on the
     /// player-built structure ghosts (Turret/Wall/Pylon) so "machines can die". DELIBERATELY a distinct tag rather
     /// than gating on bare <see cref="PlacedStructure"/>: that identity is SHARED by the reserved M7 automation
     /// machines (Harvester/Fabricator/Conveyor) whose teardown would silently drop in-flight conveyor cargo — the