ifcviewer: move const-lookup methods into ViewportCore (#84-e)

Move two pure-read methods (no GPU touch, no Qt) that the bonsai
measurement / federation-origin paths use:

  bool findInstance(uint32_t, InstanceLookup&)        const
  bool firstGeometryPointWorldM(uint32_t, Vector3d&)  const

ViewportWindow keeps both public-API method names — they now forward
to core_ for the implementation so existing callers in
bonsaiviewer/Measurement.cpp + Federation hooks don't have to change.
The InstanceLookup type also stays a `using` alias in ViewportWindow
(was added in #74).

Both methods were already de-Qt'd (`findInstance` delegates to
InstanceCompose; `firstGeometryPointWorldM` is pure Eigen). The move
is a straight transplant — no behaviour change.

Builds: desktop / bonsai / web all green. Tests 100/100.

src/ifcviewer/ViewportCore.h

@@ -41,6 +41,7 @@
 #include <unordered_map>
 
 #include "BufferPool.h"
+#include "InstanceCompose.h"
 #include "InstancedGeometry.h"
 #include "ModelGpuData.h"
 #include "StreamingThread.h"
@@ -68,6 +69,18 @@ public:
     void composeInstanceFromPlacement(InstanceCpu& inst,
                                       const ModelGpuData& m) const;
 
+    // Cross-model object_id lookup. Delegates to
+    // InstanceCompose::findInstanceInModels; the wrapper exists so
+    // callers don't have to know about the underlying map of models.
+    bool findInstance(uint32_t object_id,
+                      InstanceCompose::InstanceLookup& out) const;
+
+    // A point that actually lies on the model's first instance — used
+    // by the federation false-origin guess on first geometry. Pure
+    // read of models_gpu_; no GPU touch.
+    bool firstGeometryPointWorldM(uint32_t model_id,
+                                  Eigen::Vector3d& out) 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