feat(render): Phase A8 Wave 1 — WB scaffolding extraction + stencil low-level method

Five tasks shipped together (interdependent at build time):

Task 1: WbRenderPass enum — verbatim port of WB RenderPass.cs:1-22
Task 2: WbFrustum + WbBoundingBox + FrustumTestResult — verbatim port
  of WB Frustum.cs (98 LOC) with namespace + BoundingBox-type adaptations.
  +7 unit tests.
Task 3: EnvCellSceneryInstance + EnvCellLandblock — verbatim port of WB
  SceneryInstance.cs:1-161, renamed scope-narrow. Dropped editor-only
  fields (DisqualificationReason, ParticleEmitters, IsQueuedForUpload,
  InstanceBufferOffset, InstanceCount, MdiCommands, IsTransformOnlyUpdate)
  + InstanceId narrowed uint (we don't use ObjectId's editor methods).
  +5 unit tests.
Task 4: EnvCellVisibilitySnapshot — direct port of WB VisibilitySnapshot
  narrowed to BatchedByCell + VisibleLandblocks only.
Task 7: IndoorCellStencilPipeline.RenderBuildingStencilMask — new
  low-level WB-faithful entry mirroring PortalRenderManager:471-484.
  No surrounding GL state setup (caller's responsibility). Probe fields
  LastStencilVertexCount / LastStencilWasFarPunch / LastStencilBuildingId
  for the [stencil] probe emitter in Task 9.

Build green, 18 tests pass (7 new Frustum + 5 new SceneryInstance + 6
existing stencil pipeline). Ready for Wave 2 (EnvCellRenderer port).

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

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

@@ -0,0 +1,31 @@
+namespace AcDream.App.Rendering.Wb;
+
+/// <summary>
+/// Phase A8 (2026-05-28): WB's RenderPass enum, extracted verbatim from
+/// <c>references/WorldBuilder/Chorizite.OpenGLSDLBackend/Lib/RenderPass.cs:1-22</c>.
+///
+/// Renamed to <c>WbRenderPass</c> to keep the WB-faithful name distinct
+/// from any future acdream-side RenderPass with different semantics.
+/// Consumed by <see cref="EnvCellRenderer"/> and matches the
+/// <c>uRenderPass</c> uniform in the modern mesh shaders.
+/// </summary>
+public enum WbRenderPass
+{
+    /// <summary>
+    /// The opaque pass. Only non-transparent objects are rendered.
+    /// </summary>
+    Opaque = 0,
+
+    /// <summary>
+    /// The transparent pass. Only transparent objects are rendered,
+    /// usually after the opaque pass.
+    /// </summary>
+    Transparent = 1,
+
+    /// <summary>
+    /// A single-pass render that includes both opaque and (sometimes)
+    /// transparent objects, or for special cases like skyboxes and
+    /// certain UI elements.
+    /// </summary>
+    SinglePass = 2,
+}