test: /fp:precise reproduced the residue byte for byte — instrument instead of guessing

An /fp:precise build returned the identical 454 samples, identical max delta,
identical coordinate. A different float model producing byte-identical output is
proof that rounding is not the cause, so the /fp:fast explanation is dead. That
is three failed hypotheses on one discrepancy (FVector round-trip, then "check
the roughness window", then /fp:fast), each reasoned from plausibility and each
costing a build.

So: stop reasoning, print. FVoxelOpStack::EvalSample exposes the full sample, and
MazeEquivalence now dumps the worst point in raw hex -- both densities, the
stack's internal SDF, and the carve factor reconstructed from each side. The
recovered carve localises it: identical carve + differing density means the fault
is after the conversion; differing carve means it is in the SDF (lattice edges or
VoxelSDF::Capsule) or in SmoothStep01.

Note for whoever reads the docs next: AUDIT C9 and OPSTACK-PLAN 2.6 currently
assert the /fp:fast story as the explanation for THIS residue. That specific
claim is falsified and needs walking back once the dump identifies the real
cause. C9's other half -- that UBT's FP default differs by toolchain and the MP
model assumes bit-reproducible terrain -- stands independently; it was read out
of VCToolChain.cs and ClangToolChain.cs, not inferred from this test.

Phase 1 step 3 (wiring the stack into GetDensityAt) is paused until this is
understood. Small unexplained numeric differences do not get smaller when you
build on top of them.

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

Source/VoxelForge/Public/VoxelDensityOpStack.h

@@ -83,10 +83,19 @@ public:
      * Returns INTERNAL-convention density (positive = solid). The caller negates once for MC.
      */
     float EvalInternal(float WorldX, float WorldY, float WorldZ) const
+    {
+        return EvalSample(WorldX, WorldY, WorldZ).Density;
+    }
+
+    /** L'état COMPLET (densité + SDF) après toute la pile. Diagnostic : quand une comparaison
+     *  avec l'ancien chemin diverge, c'est le canal SDF qui dit si l'écart naît avant ou après
+     *  la conversion. / The full state after the stack — the SDF channel is what says whether a
+     *  divergence is born before or after the carve. */
+    FVoxelOpSample EvalSample(float WorldX, float WorldY, float WorldZ) const
     {
         FVoxelOpSample S;
         for (const TUniquePtr<IVoxelDensityOp>& Op : Ops) { Op->Eval(WorldX, WorldY, WorldZ, S); }
-        return S.Density;
+        return S;
     }
 
     /** Le même, négaté pour le mesher (négatif = solide). */