build: FPSemantics = Precise + clear the IWYU debt that blocked it

Jahni: cross-platform play (Linux and Windows, either side hosting) is a product
requirement, so fix C9 at the cause instead of measuring the symptom.

Verified in UE 5.7 source rather than assumed:
  VCToolChain.cs:1264    Default -> /fp:fast          Precise -> /fp:precise
  ClangToolChain.cs:712  Default -> -ffp-contract=off Precise -> -ffp-contract=off
Default really did mean opposite float models per platform; Precise collapses them
onto the same one, so a Windows host and a Linux client agree by construction.

Losing the shared PCH is what the IWYU debt hid behind. Every use turned out to be a
pointer, TWeakObjectPtr or TSubclassOf parameter, so forward declarations suffice;
only the templates and macros needed real includes. Seven headers fixed.

VoxelDensityVolume.h was the one worth catching: it tests ENABLE_DRAW_DEBUG in an
#if, and an undefined macro there is silently 0 — the debug block would have
vanished without a warning rather than failing the build. Include paths verified
against the engine tree, not guessed.

Expect a residual tail; the shared PCH hid these for years and only a build
enumerates them all. Build.cs now says so, and says the fix is to add the include
rather than revert FPSemantics.

Also adds VoxelForge.Determinism.CrossPlatformDigest: SHAPE digest (sign of density
= the world) and FIELD digest (bit-for-bit) over a fixed integer grid, plus NearIso
to bound how many samples could flip sign at all. Reports rather than asserts until
pinned. The cross-platform comparison itself is deferred per Jahni.

Expect a perf regression from losing reassociation and contraction on a noise-heavy
hot path — measure against ARCHITECTURE 8.10.

UNVERIFIED: not compiled.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Source/VoxelForge/Public/VoxelSettings.h

@@ -11,6 +11,9 @@
 #include "VoxelStrateDefinition.h"
 #include "VoxelSettings.generated.h"
 
+// IWYU : pointeur seulement (VoxelMaterial). / Pointer-only use.
+class UMaterialInterface;
+
 UCLASS(BlueprintType)
 class UVoxelSettings : public UPrimaryDataAsset
 {