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;
+        }
+    }
+}

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

src/ifcviewer/ViewportWindow.cpp

@@ -638,7 +638,8 @@ ViewportWindow::ViewportWindow(QWindow* parent)
       streaming_thread_(core_.streaming_thread_),
       models_gpu_     (core_.models_gpu_),
       next_model_id_  (core_.next_model_id_),
-      next_object_id_ (core_.next_object_id_) {
+      next_object_id_ (core_.next_object_id_),
+      federated_false_origin_meters_(core_.federated_false_origin_meters_) {
     // wgpu doesn't need a GL context; we just need a real native window
     // whose backing layer matches the GPU API wgpu will drive.
     //
@@ -1377,37 +1378,7 @@ void ViewportWindow::setModelTransformation(uint32_t model_id,
     recomposeAndUploadModel(model_id);
 }
 
-void ViewportWindow::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;
-        }
-    }
-}
+// composeInstanceFromPlacement moved to ViewportCore (#84-d).
 
 void ViewportWindow::recomposeAndUploadModel(uint32_t model_id) {
     if (!wgpu_initialized_) return;
@@ -1419,7 +1390,7 @@ void ViewportWindow::recomposeAndUploadModel(uint32_t model_id) {
     std::vector<InstanceGpu> gpu(m.instances.size());
     for (size_t i = 0; i < m.instances.size(); ++i) {
         InstanceCpu& inst = m.instances[i];
-        composeInstanceFromPlacement(inst, m);
+        core_.composeInstanceFromPlacement(inst, m);
 
         InstanceGpu& dst = gpu[i];
         std::memcpy(dst.transform, inst.transform, sizeof(dst.transform));

src/ifcviewer/ViewportWindow.h

@@ -624,8 +624,9 @@ private:
     // in double; the cast to float happens last so large IFC placements
     // get cancelled by the federation false origin before precision is
     // narrowed. Mirrors GL ViewportWindow::composeInstanceFromPlacement.
-    void composeInstanceFromPlacement(InstanceCpu& inst,
-                                      const ModelGpuData& m) const;
+    // Implementation lives in ViewportCore now (#84-d); this declaration
+    // stayed during the move and forwards to core_ — once every internal
+    // caller routes through ViewportCore directly the forwarder goes away.
 
     // Walk every instance of `model_id`, recompose its transform from the
     // current federation matrices, refresh per-chunk world AABBs, and
@@ -1120,7 +1121,8 @@ private:
     // FederatedFalseOrigin matrix, in metres. Default identity. Stored
     // but not yet applied to per-instance composed transforms — the
     // recompose pass arrives with the federation-load OOM work.
-    Eigen::Matrix4d federated_false_origin_meters_ = Eigen::Matrix4d::Identity();
+    // Federation false-origin alias (storage in core_).
+    Eigen::Matrix4d& federated_false_origin_meters_;
 
     // Per-frame LOD selection counts, mutated from cullModelCpuCompute
     // and reset after the [frame] heartbeat prints them. Keeps an eye