fix(render): eliminate EnvCell identity collisions

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

@@ -120,7 +120,9 @@ public sealed class EnvCellLandblockBuildBuilder
     }
 
     /// <summary>
-    /// Source: WB EnvCellRenderManager.cs:94-103 (verbatim arithmetic).
+    /// Collision-resistant successor to WB EnvCellRenderManager.cs:94-103.
+    /// The legacy polynomial and its installed-DAT collision remain covered by
+    /// Core conformance tests.
     /// </summary>
     public static ulong ComputeGeometryId(
         uint environmentId,

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

@@ -4,7 +4,7 @@
 // four reverted RR7 variants couldn't fix.
 //
 // Sources ported byte-for-byte:
-//   GetEnvCellGeomId      <- WB EnvCellRenderManager.cs:94-103
+//   GetEnvCellGeomId      <- collision-resistant successor to WB's content key
 //   PrepareRenderBatches  <- WB EnvCellRenderManager.cs:247-373
 //   Render(filter:)       <- WB EnvCellRenderManager.cs:395-511
 //   RenderModernMDIInternal <- WB BaseObjectRenderManager.cs:709-848 (single-slot variant)
@@ -353,14 +353,15 @@ public sealed unsafe class EnvCellRenderer : IDisposable
 
     // ---------------------------------------------------------------------------
     // GetEnvCellGeomId
-    // Verbatim copy of WB EnvCellRenderManager.cs:94-103.
+    // Shared collision-resistant content identity. Core retains WB's original
+    // polynomial for conformance evidence and the installed-DAT collision test.
     // ---------------------------------------------------------------------------
 
     /// <summary>
     /// Returns a deduplicated geometry ID for an EnvCell based on its environment,
     /// cell structure index, and surface IDs. Bit 33 is set to distinguish from
     /// per-cell IDs (which use bit 32).
-    /// Source: WB EnvCellRenderManager.cs:94-103 (verbatim).
+    /// See <see cref="AcDream.Core.Rendering.Wb.EnvCellGeometryIdentity"/>.
     /// </summary>
     public static ulong GetEnvCellGeomId(uint environmentId, ushort cellStructure, List<ushort> surfaces)
         => EnvCellLandblockBuildBuilder.ComputeGeometryId(

src/AcDream.Core/Rendering/Wb/EnvCellGeometryIdentity.cs

@@ -3,21 +3,52 @@ using System.Collections.Generic;
 namespace AcDream.Core.Rendering.Wb;
 
 /// <summary>
-/// Computes WorldBuilder's content identity for an EnvCell shell.
+/// Computes acdream's collision-resistant content identity for an EnvCell shell.
 /// Equal environment, cell-structure, and ordered surface tuples share one
 /// prepared geometry payload even when they occur under different cell IDs.
 /// </summary>
 public static class EnvCellGeometryIdentity
 {
+    private const ulong FnvOffsetBasis = 14695981039346656037UL;
+    private const ulong FnvPrime = 1099511628211UL;
+    private const ulong PayloadMask = 0x0FFF_FFFD_FFFF_FFFFUL;
+    private const ulong Namespace = 0xE000_0002_0000_0000UL;
+
     /// <summary>
-    /// Source: WorldBuilder <c>EnvCellRenderManager.GetEnvCellGeomId</c>,
-    /// lines 94-103. Bit 33 separates content identities from per-cell IDs,
-    /// whose synthetic geometry requests use bit 32.
+    /// Hashes the complete typed tuple with FNV-1a and places it in a dedicated
+    /// high-bit namespace. Bit 33 remains set for compatibility with existing
+    /// synthetic-geometry diagnostics; the top nibble prevents overlap with
+    /// DAT IDs and older low-bit synthetic IDs.
+    ///
+    /// WorldBuilder's original 31× polynomial is retained in
+    /// <see cref="ComputeLegacyWorldBuilder"/> for conformance evidence, but it
+    /// is not a valid resource identity: installed retail DAT cells 0x00030175
+    /// and 0x01BC0105 produce the same value for different geometry tuples.
     /// </summary>
     public static ulong Compute(
         uint environmentId,
         ushort cellStructure,
         IReadOnlyList<ushort> surfaces)
+    {
+        ulong hash = FnvOffsetBasis;
+        AddUInt32(ref hash, environmentId);
+        AddUInt16(ref hash, cellStructure);
+        AddUInt32(ref hash, checked((uint)surfaces.Count));
+        for (int i = 0; i < surfaces.Count; i++)
+            AddUInt16(ref hash, surfaces[i]);
+
+        return (hash & PayloadMask) | Namespace;
+    }
+
+    /// <summary>
+    /// Verbatim WorldBuilder <c>EnvCellRenderManager.GetEnvCellGeomId</c>
+    /// lines 94-103. Kept only to demonstrate and regression-test the real DAT
+    /// collision that required the stronger internal identity.
+    /// </summary>
+    public static ulong ComputeLegacyWorldBuilder(
+        uint environmentId,
+        ushort cellStructure,
+        IReadOnlyList<ushort> surfaces)
     {
         long hash = 17;
         hash = unchecked(hash * 31 + environmentId);
@@ -26,4 +57,24 @@ public static class EnvCellGeometryIdentity
             hash = unchecked(hash * 31 + surfaces[i]);
         return (ulong)hash | 0x2_0000_0000UL;
     }
+
+    private static void AddUInt16(ref ulong hash, ushort value)
+    {
+        AddByte(ref hash, (byte)value);
+        AddByte(ref hash, (byte)(value >> 8));
+    }
+
+    private static void AddUInt32(ref ulong hash, uint value)
+    {
+        AddByte(ref hash, (byte)value);
+        AddByte(ref hash, (byte)(value >> 8));
+        AddByte(ref hash, (byte)(value >> 16));
+        AddByte(ref hash, (byte)(value >> 24));
+    }
+
+    private static void AddByte(ref ulong hash, byte value)
+    {
+        hash ^= value;
+        hash = unchecked(hash * FnvPrime);
+    }
 }