Portal made visible: cyan beacon pillar + always-on RoomExplore steer prompt (DR-046 follow-up)

The room-exit portal was invisible in the first playtest - players loitered
out the 30s ExploreGrace timeout. Adds RegionMath.ExpeditionPortalPos as the
single client-derivable portal-position authority (HUD prompt + beacon can't
drift), a pooled breathing HDR pillar in CombatFeedbackSystem shown only
during RoomExplore, and an in-range/out-of-range two-stage prompt.

Also repairs the comment-swallowed _portalMat initialization (NRE in
OnStartRunning killed ALL combat feedback each session and auto-paused the
editor via Error Pause). 456/456 EditMode.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/_Project/Scripts/Simulation/World/RegionComponents.cs

@@ -53,7 +53,18 @@ namespace ProjectM.Simulation
             return baseCenter + new float3(ExpeditionOffsetX + subSlot * RoomStrideX, 0f, 0f);
         }
 
-        /// <summary>World-space origin of <paramref name="region"/>, given the base center (BaseGridMath.PlotCenter).
+        /// <summary>World-space position of the room-exit PORTAL for sub-slot <paramref name="subSlot"/> — the single
+        /// client-derivable authority the HUD prompt AND the presentation beacon both resolve through (DR-046), so they
+        /// can't drift. = the room origin nudged by <see cref="Tuning.PortalOffsetZ"/> in Z.</summary>
+        public static float3 ExpeditionPortalPos(float3 baseCenter, byte subSlot)
+        {
+            float3 p = ExpeditionRoomOrigin(baseCenter, subSlot);
+            p.z += Tuning.PortalOffsetZ;
+            return p;
+        }
+
+        
+/// <summary>World-space origin of <paramref name="region"/>, given the base center (BaseGridMath.PlotCenter).
         /// The expedition resolves to room sub-slot 0 (legacy call sites; room-aware systems pass the ACTIVE
         /// sub-slot to <see cref="ExpeditionRoomOrigin"/> directly).</summary>
         public static float3 RegionOrigin(byte region, float3 baseCenter)