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>

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)