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>

Standalone/Core/src/CoreNamespacePreamble.h

@@ -54,3 +54,40 @@
 // value tree shared lets the viewer resolve its reference world and Core snapshot from one parse.
 #include "Json.h"
 #include "VoxelForgeCore/VoxelForgeCore.h"
+
+// Keep Core's expression-level FMA behavior fixed regardless of the toolchain setting.
+// Clang's "fast" mode ignores the pragma, so Core's runtime self-test rejects that build.
+#if defined(__clang__)
+    #pragma clang fp contract(off)
+#elif defined(_MSC_VER)
+    #pragma fp_contract(off)
+#elif defined(__GNUC__)
+    #pragma GCC optimize ("fp-contract=off")
+#else
+    #error "VoxelForge Core requires a compiler-specific FP contraction control"
+#endif
+
+namespace vfcore::fp_contract
+{
+#if defined(_MSC_VER)
+    #define VFCORE_FP_PROBE_NOINLINE __declspec(noinline)
+#else
+    #define VFCORE_FP_PROBE_NOINLINE __attribute__((noinline))
+#endif
+
+    [[maybe_unused]] static VFCORE_FP_PROBE_NOINLINE double multiplyAddProbe(
+        double a, double b, double c) noexcept
+    {
+        return a * b + c;
+    }
+
+    [[maybe_unused]] static bool selfTest() noexcept
+    {
+        volatile double a = 1.0 + 0x1p-27;
+        volatile double b = 1.0 - 0x1p-27;
+        volatile double c = -1.0;
+        return multiplyAddProbe(a, b, c) == 0.0;
+    }
+
+    #undef VFCORE_FP_PROBE_NOINLINE
+}

Standalone/Core/src/VoxelForgeCore.cpp

@@ -969,6 +969,11 @@ namespace VoxelForgeCore
     bool World::compile(const ShapeBuildRequest& request,const JobScheduler& scheduler,std::string& error)
     {
         if(!impl_->owner){error="resolved world snapshot is empty";return false;}
+        if(!::vfcore::fp_contract::selfTest())
+        {
+            error="Core startup refused: floating-point contraction self-test detected a fused multiply-add; this compiler/flag combination is unsupported for world generation";
+            return false;
+        }
         auto shape=std::make_unique<FStandaloneShapeWorld>();
         const bool ok=shape->Build(*impl_->owner,ToNative(request.minVoxel),ToNative(request.maxVoxel),
             ToNative(request.clipMinVoxel),ToNative(request.clipMaxVoxel),