phase(N.4): WbFoundationFlag scaffold for ACDREAM_USE_WB_FOUNDATION env var

Creates the src/AcDream.App/Rendering/Wb/ folder and the static flag
gate that other call sites will import. Read once at static-init time.
Set ACDREAM_USE_WB_FOUNDATION=1 to enable WB foundation routing.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/AcDream.App/Rendering/Wb/WbFoundationFlag.cs

@@ -0,0 +1,22 @@
+namespace AcDream.App.Rendering.Wb;
+
+/// <summary>
+/// Process-lifetime cache of <c>ACDREAM_USE_WB_FOUNDATION</c> env var.
+/// Read once at static-init time; all consumers import this rather than
+/// re-reading the env var per call (env-var lookups on Windows are not
+/// free at hot-path cadence).
+///
+/// <para>
+/// Set <c>ACDREAM_USE_WB_FOUNDATION=1</c> to route static-scenery + atlas
+/// content through WB's <c>ObjectMeshManager</c>; per-instance customized
+/// content (server <c>CreateObject</c> entities) takes the existing
+/// <see cref="TextureCache.GetOrUploadWithPaletteOverride"/> path either
+/// way. Flag becomes default-on at end of Phase N.4 after visual
+/// verification.
+/// </para>
+/// </summary>
+public static class WbFoundationFlag
+{
+    public static bool IsEnabled { get; } =
+        System.Environment.GetEnvironmentVariable("ACDREAM_USE_WB_FOUNDATION") == "1";
+}