fix(portal): synchronize destination presentation state

src/AcDream.Core/World/ProceduralSceneryIdAllocator.cs

@@ -0,0 +1,39 @@
+namespace AcDream.Core.World;
+
+/// <summary>
+/// Allocates stable, collision-free ids for procedurally generated scenery.
+///
+/// <para>
+/// The top nibble is fixed at <c>0x8</c>, so bit 31 continues to identify
+/// procedural scenery to every renderer and physics consumer. The remaining
+/// 28 bits are X(8), Y(8), and a 12-bit per-landblock counter:
+/// <c>0x8XXYYIII</c>. No consumer decodes the former byte-aligned
+/// <c>0x80XXYYII</c> layout; they classify only on bit 31.
+/// </para>
+///
+/// <para>
+/// The old 8-bit counter aborted generation after 256 drawable spawns. That
+/// exception rejected the entire streamed landblock before render publication,
+/// exposing the grey clear color as portal space ended. A 12-bit counter gives
+/// 4096 entries while retaining full landblock coordinates. Overflow fails
+/// before it can alias the next landblock.
+/// </para>
+/// </summary>
+public static class ProceduralSceneryIdAllocator
+{
+    public const uint MaxCounter = 0xFFFu;
+
+    public static uint Base(uint landblockX, uint landblockY)
+        => 0x80000000u
+           | ((landblockX & 0xFFu) << 20)
+           | ((landblockY & 0xFFu) << 12);
+
+    public static uint Allocate(uint landblockX, uint landblockY, ref uint counter)
+    {
+        if (counter > MaxCounter)
+            throw new InvalidDataException(
+                $"Landblock ({landblockX & 0xFFu:X2},{landblockY & 0xFFu:X2}) exceeds the 4096-entry procedural scenery id namespace.");
+
+        return Base(landblockX, landblockY) + counter++;
+    }
+}