ifcviewer: move composeInstanceFromPlacement into ViewportCore (#84-d)

First method-body migration. composeInstanceFromPlacement composes the
federated-false-origin × model-transformation × coordinate-operation ×
placement chain and re-derives the world AABB; it's a small,
self-contained method that only reads scene state and one matrix.

Moved:
  Eigen::Matrix4d federated_false_origin_meters_   (storage → core_)
  void composeInstanceFromPlacement(InstanceCpu&, ...) (body → core_)

ViewportWindow keeps:
  - alias reference to federated_false_origin_meters_ (existing
    setFederatedFalseOrigin call site still writes through it)
  - no method declaration — internal callers route through core_

Internal caller (recomposeAndUploadModel) now invokes
core_.composeInstanceFromPlacement; once recomposeAndUploadModel
itself moves into ViewportCore the call shortens back.

Pattern for the rest of #84: state moves, then method body moves,
then internal callers update. Each commit leaves desktop / bonsai /
web green and tests 100/100. This is one of many such steps.

src/ifcviewer/ViewportCore.cpp

@@ -19,5 +19,39 @@
 
 #include "ViewportCore.h"
 
+#include "InstanceCompose.h"
+
 ViewportCore::ViewportCore(ViewportHost* host) : host_(host) {}
 ViewportCore::~ViewportCore() = default;
+
+void ViewportCore::composeInstanceFromPlacement(InstanceCpu& inst,
+                                                const ModelGpuData& m) const {
+    if (inst.mesh_id < m.meshes.size()) {
+        const MeshInfo& mi = m.meshes[inst.mesh_id];
+        InstanceCompose::composeInstance(
+            inst.placement_transformation,
+            federated_false_origin_meters_,
+            m.model_transformation_meters,
+            m.coordinate_operation_meters,
+            mi.local_aabb_min, mi.local_aabb_max,
+            inst.transform,
+            inst.world_aabb_min, inst.world_aabb_max);
+    } else {
+        // Unknown mesh id: still compose the transform (downstream may
+        // use it for picking / readback even without geometry), but
+        // emit a degenerate world AABB so cull doesn't pick this up.
+        const float zero[3] = {0.0f, 0.0f, 0.0f};
+        InstanceCompose::composeInstance(
+            inst.placement_transformation,
+            federated_false_origin_meters_,
+            m.model_transformation_meters,
+            m.coordinate_operation_meters,
+            zero, zero,
+            inst.transform,
+            inst.world_aabb_min, inst.world_aabb_max);
+        for (int a = 0; a < 3; ++a) {
+            inst.world_aabb_min[a] = 0.0f;
+            inst.world_aabb_max[a] = 0.0f;
+        }
+    }
+}