wip(assembler): round L3 -- Core forces FP contraction off and refuses to generate if it can't

CoreNamespacePreamble.h applies per-compiler contraction-off pragmas (Clang, MSVC, GCC; unknown
compiler = #error) and a fused-multiply-add self-test that World::compile runs before building.
Clang -ffp-contract=on and GCC's default (fast) now reproduce the L1/Windows hashes (they diverged
in L2); Clang -ffp-contract=fast ignores the pragma and is refused at startup (negative control).
Seed sweep with every cube on real terrain: 48 seeds x 2 recipes x 3 builds x W1/W12, 576 runs,
0 mismatches, 0 empty meshes. Cost +0.5% (2.326 -> 2.337 s, interleaved). World unchanged.
MSVC/UBT not compiled here: Windows to-do list in Linux3/REPORT.md.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CODEMAP.md

@@ -158,6 +158,7 @@ remains `0`, and lateral regions remain gated off.
 | `VoxelForge.Build.cs` | Deps: Core, CoreUObject, Engine, **GameplayTags**, **RealtimeMeshComponent**, `VoxelForgeCore`. `AVoxelWorld::bUseVoxelForgeCore` opts into the Core runtime route; it defaults off. |
 | `../../VoxelForgeCore/VoxelForgeCore.Build.cs` | C++20 runtime module. Reads `Standalone/Core/core_sources.txt`, validates one include shim per canonical source, and fails UBT on missing, extra, or mismatched shims. No unity/PCH; private copied implementation stays under `vfcore::detail`. |
 | `../../VoxelForgeCore/Private/Manifest/VFCore_*.cpp` | Eighteen one-source UBT shims that include canonical files under `Standalone/`; they do not copy Core implementation. The API-boundary row includes `Core/src/VoxelForgeCore.cpp` directly. |
+| `../../Standalone/Core/src/CoreNamespacePreamble.h` | First include of every Core TU. Forces FP contraction off per compiler (Clang/MSVC/GCC pragmas; unknown compiler = `#error`) and defines `vfcore::fp_contract::selfTest`, which `World::compile` runs before building: a fused multiply-add (e.g. Clang `-ffp-contract=fast`) refuses world generation. The world depends on contraction (Linux2/Linux3 reports), so never remove either. |
 | `../../VoxelForgeCore/Private/ModuleEntry/` | Unreal module entry and an isolated `CoreMinimal.h` shim for the actual engine header. |
 | `../../Standalone/Core/include/VoxelForgeCore/VoxelForgeCore.h` | Standard-library-only public Core API. `VOXELFORGECORE_API` exports the API from the Unreal module and is empty for standalone builds. |
 | `../../Standalone/Core/src/FloatingIslandsInternal.h` | Canonical standalone/Core floating-island planner, geometry-versioned field, manifest serializer, influence bounds, capsule-route checks, and thickness audit. |