phase(N.5) Task 9 fixup: layout assertion + DrawCommandStride const

Code quality review caught:
- sizeofDEIC was a local; promoted to public const DrawCommandStride
  so tests can reference it symbolically.
- BatchDataPublic layout invariant (size + field offsets) wasn't
  asserted in tests. Added BatchDataPublic_LayoutMatchesPrivateBatchData
  + DrawCommandStride_MatchesStructSize tests to gate Task 10's
  MemoryMarshal.Cast<BatchData, BatchDataPublic> safety.
- Plan doc updated: BatchDataPublic spec was Pack=4 (wrong — must
  match private BatchData's Pack=8 for the cast to work). Implementation
  was already correct; plan now matches.

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

docs/superpowers/plans/2026-05-08-phase-n5-modern-rendering.md

@@ -1364,7 +1364,8 @@ public readonly record struct IndirectGroupInput(
     TranslucencyKind Translucency);
 
 /// <summary>Public mirror of the per-group BatchData laid into the SSBO. Tests verify alignment.</summary>
-[StructLayout(LayoutKind.Sequential, Pack = 4)]
+// Pack=8 (not 4) — must stay layout-identical to private BatchData for Task 10's MemoryMarshal.Cast.
+[StructLayout(LayoutKind.Sequential, Pack = 8)]
 public struct BatchDataPublic
 {
     public ulong TextureHandle;
@@ -1431,8 +1432,7 @@ public static IndirectLayoutResult BuildIndirectArrays(
         }
     }
 
-    int sizeofDEIC = 20;  // matches struct layout
-    return new IndirectLayoutResult(opaqueCount, transparentCount, opaqueCount * sizeofDEIC);
+    return new IndirectLayoutResult(opaqueCount, transparentCount, opaqueCount * DrawCommandStride);
 }
 
 private static bool IsOpaque(TranslucencyKind t)

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

@@ -534,6 +534,13 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
     // These are public so the pure-CPU unit tests in AcDream.Core.Tests can
     // exercise BuildIndirectArrays without needing a GL context.
 
+    /// <summary>
+    /// Stride in bytes of <c>DrawElementsIndirectCommand</c> in the indirect buffer.
+    /// 5 × <c>uint</c> = 20 bytes. Tests and callers reference this symbolically
+    /// rather than hard-coding <c>20</c> so a layout change produces a compile error.
+    /// </summary>
+    public const int DrawCommandStride = 20; // sizeof(DrawElementsIndirectCommand): 5 × uint
+
     /// <summary>
     /// Public view of the per-group inputs to <see cref="BuildIndirectArrays"/> — used in tests.
     /// </summary>
@@ -622,8 +629,7 @@ public sealed unsafe class WbDrawDispatcher : IDisposable
             }
         }
 
-        const int SizeofDEIC = 20; // sizeof(DrawElementsIndirectCommand) — 5 × uint
-        return new IndirectLayoutResult(opaqueCount, transparentCount, opaqueCount * SizeofDEIC);
+        return new IndirectLayoutResult(opaqueCount, transparentCount, opaqueCount * DrawCommandStride);
     }
 
     private static bool IsOpaque(TranslucencyKind t)

tests/AcDream.Core.Tests/Rendering/Wb/WbDrawDispatcherIndirectBuilderTests.cs

@@ -91,4 +91,23 @@ public sealed class WbDrawDispatcherIndirectBuilderTests
         Assert.Equal(1, result.OpaqueCount);
         Assert.Equal(0, result.TransparentCount);
     }
+
+    [Fact]
+    public void BatchDataPublic_LayoutMatchesPrivateBatchData()
+    {
+        // Task 10 will use MemoryMarshal.Cast<BatchData, BatchDataPublic> to
+        // expose the dispatcher's per-frame BatchData[] scratch to BuildIndirectArrays
+        // without copying. The cast is only safe if the structs have identical
+        // layout (size, field offsets). Both use [StructLayout(Sequential, Pack=8)].
+        Assert.Equal(16, System.Runtime.CompilerServices.Unsafe.SizeOf<WbDrawDispatcher.BatchDataPublic>());
+        Assert.Equal(0,  (int)System.Runtime.InteropServices.Marshal.OffsetOf<WbDrawDispatcher.BatchDataPublic>(nameof(WbDrawDispatcher.BatchDataPublic.TextureHandle)));
+        Assert.Equal(8,  (int)System.Runtime.InteropServices.Marshal.OffsetOf<WbDrawDispatcher.BatchDataPublic>(nameof(WbDrawDispatcher.BatchDataPublic.TextureLayer)));
+        Assert.Equal(12, (int)System.Runtime.InteropServices.Marshal.OffsetOf<WbDrawDispatcher.BatchDataPublic>(nameof(WbDrawDispatcher.BatchDataPublic.Flags)));
+    }
+
+    [Fact]
+    public void DrawCommandStride_MatchesStructSize()
+    {
+        Assert.Equal(WbDrawDispatcher.DrawCommandStride, System.Runtime.CompilerServices.Unsafe.SizeOf<DrawElementsIndirectCommand>());
+    }
 }