Major pipeline changes to map building, introducing 3player map, and adding Shared build mode to level authoring options

Assets/_Project/Scripts/Gameplay/LevelLoader.cs

@@ -355,6 +355,39 @@ namespace TD.Gameplay
             return level.OwnerGrid[idx];
         }
 
+        /// <summary>
+        /// This map's build-permission rule. <see cref="BuildAccess.OwnerOnly"/> for an unloaded
+        /// or unbaked level, which is the safe default (it can only ever reject more).
+        /// </summary>
+        public BuildAccess BuildAccess => level != null ? level.BuildAccess : BuildAccess.OwnerOnly;
+
+        /// <summary>
+        /// True if <paramref name="slot"/> is permitted to build on <paramref name="tile"/> under
+        /// this map's <see cref="BuildAccess"/> rule.
+        /// </summary>
+        /// <remarks>
+        /// <para>Permission ONLY — the caller still has to check <see cref="GetPlacement"/> and
+        /// <see cref="IsOccupied"/>. Keeping them separate is deliberate: each produces a different
+        /// <c>PlacementRejectionReason</c>, and collapsing them would cost the player a useful
+        /// message.</para>
+        ///
+        /// <para>Both the client's ghost preview and the server's authoritative check call this, so
+        /// they cannot drift apart and show a white ghost that then gets rejected.</para>
+        ///
+        /// <para>Under <see cref="BuildAccess.Shared"/> the tile must still belong to SOME player
+        /// zone. In practice every Buildable tile does, but stating it here keeps the method
+        /// meaningful on its own rather than relying on a caller's later check.</para>
+        /// </remarks>
+        public bool HasBuildPermission(Vector2Int tile, PlayerSlot slot)
+        {
+            if (slot == PlayerSlot.None) return false;
+
+            PlayerSlot owner = GetOwner(tile);   // PlayerSlot.None when out of bounds
+            return BuildAccess == BuildAccess.Shared
+                ? owner != PlayerSlot.None
+                : owner == slot;
+        }
+
         /// <summary>
         /// True if <paramref name="tile"/> is currently occupied by a placed tower footprint.
         /// Returns false for out-of-bounds tiles. Unlike <see cref="IsWalkable"/>, this grid