Preserve precise viewer placements

Keep placement transformations in double precision through streaming, sidecar caching, and viewport recomposition so large coordinates can be cancelled before the final GPU float upload.

Generated with the assistance of an AI coding tool.

src/ifcviewer/InstancedGeometry.h

@@ -101,17 +101,20 @@ static_assert(sizeof(InstanceGpu) == 80, "InstanceGpu must be 80 bytes");
 //
 // `placement_transformation` is the raw streamer output (the iterator's
 // transform with vertex-rebasing offset folded in; pre-CoordinateOperation
-// / FederatedFalseOrigin / ModelTransformation).  `transform` is the
-// composed FederatedFalseOrigin · ModelTransformation · CoordinateOperation
-// · placement_transformation result — what gets uploaded to the SSBO and
-// used to compute world_aabb_*.  When ViewportWindow's stage matrices are
-// all identity (default), the two are equal.
+// / FederatedFalseOrigin / ModelTransformation).  Keep it in double precision:
+// large IFC placements must not be rounded before the federation false origin
+// has a chance to cancel them.  `transform` is the composed
+// FederatedFalseOrigin · ModelTransformation · CoordinateOperation
+// · placement_transformation result — narrowed to float only after composition,
+// uploaded to the SSBO, and used to compute world_aabb_*.  When ViewportWindow's
+// stage matrices are all identity (default), transform is the float rendering
+// copy of placement_transformation.
 struct InstanceCpu {
     uint32_t mesh_id                  = 0;  // index into meshes array
     uint32_t object_id                = 0;
     uint32_t color_override_rgba8     = 0;
     uint32_t model_id                 = 0;
-    float    placement_transformation[16]{};
+    double   placement_transformation[16]{};
     float    transform[16]{};
     float    world_aabb_min[3]{};
     float    world_aabb_max[3]{};
@@ -139,7 +142,7 @@ struct InstanceChunk {
     uint32_t local_mesh_id        = 0;
     uint32_t object_id            = 0;
     uint32_t color_override_rgba8 = 0;
-    float    transform[16]{};
+    double   transform[16]{};
     float    world_aabb_min[3]{};
     float    world_aabb_max[3]{};
 };