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.h

@@ -35,10 +35,13 @@
 
 #include <webgpu/webgpu.h>
 
+#include <Eigen/Dense>
+
 #include <cstdint>
 #include <unordered_map>
 
 #include "BufferPool.h"
+#include "InstancedGeometry.h"
 #include "ModelGpuData.h"
 #include "StreamingThread.h"
 #include "ViewportHost.h"
@@ -53,6 +56,18 @@ public:
 
     ViewportHost* host() const { return host_; }
 
+    // ---- Scene-mutation methods --------------------------------------------
+    //
+    // composeInstanceFromPlacement composes the per-instance
+    //   transform = federated_false_origin × model_transformation
+    //             × coordinate_operation × placement
+    // (all in metres, double precision) and rebakes the world AABB
+    // from the mesh-local one. Used by the per-model recompose path
+    // after any of the four federation matrices change. Pure scene
+    // math — no GPU touch.
+    void composeInstanceFromPlacement(InstanceCpu& inst,
+                                      const ModelGpuData& m) const;
+
     // Friend access for ViewportWindow's reference proxies. As each
     // render method moves into ViewportCore it stops needing these
     // (it touches the fields directly); once everything has migrated
@@ -129,6 +144,11 @@ private:
     // the sidecar's local object_ids by base_object_id_so_far so picks
     // are unambiguous across models.
     uint32_t next_object_id_ = 1;
+
+    // Federation false origin (metres, double precision). Applied to
+    // every instance composition so geometry rebased through a large
+    // model offset doesn't lose float32 precision near the GPU origin.
+    Eigen::Matrix4d federated_false_origin_meters_ = Eigen::Matrix4d::Identity();
 };
 
 #endif  // VIEWPORTCORE_H