phase(N.1): add ACDREAM_USE_WB_SCENERY feature flag scaffold

Phase N.1 step 2: read the env var into a static bool. No behavior
change yet — the flag is consumed in step 5 when GenerateViaWb is
wired in.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.Core/World/SceneryGenerator.cs

@@ -36,6 +36,16 @@ public static class SceneryGenerator
     private const float CellSize = 24.0f;
     private const float LandblockSize = 192.0f;  // 8 cells * 24 units
 
+    /// <summary>
+    /// Phase N.1 feature flag — when set to "1", scenery placement uses
+    /// WorldBuilder's <c>SceneryHelpers</c> + <c>TerrainUtils</c> instead of
+    /// our hand-ported algorithms. Default off until visual verification at
+    /// landblock 0xA9B1 confirms behavior. See
+    /// <c>docs/superpowers/specs/2026-05-08-phase-n1-scenery-via-wb-helpers-design.md</c>.
+    /// </summary>
+    internal static readonly bool UseWbScenery =
+        System.Environment.GetEnvironmentVariable("ACDREAM_USE_WB_SCENERY") == "1";
+
     public readonly record struct ScenerySpawn(
         uint ObjectId,        // GfxObj or Setup id
         Vector3 LocalPosition, // landblock-local world units