ifcviewer: upload per-instance world AABBs to a GPU SSBO

Scaffolding for Phase 3E (GPU compute cull).  After finalizeModel /
applyCachedModel, pack each InstanceCpu's world AABB + mesh_id +
reflection bit into a std430-friendly 32 B record and push it to a
per-model aabb_ssbo.  No consumer yet — the CPU cull still drives
rendering — but the next commits will point a compute shader at this
buffer and have it produce the visible list + indirect commands
directly on the GPU.

Cost: 32 B per instance, ~18 MB for the 569 k-instance test scene.
One-shot upload at finalize time; streaming-time appends aren't
mirrored (the CPU cull doesn't need the SSBO, and finalizeModel
rebuilds the whole thing in one go).

src/ifcviewer/ViewportWindow.h

@@ -94,6 +94,14 @@ struct ModelGpuData {
     std::vector<BvhItem> bvh_items;
     ModelBvh             bvh;
 
+    // Per-instance world AABB on the GPU, 1:1 with `instances`.
+    // Populated at finalize / applyCachedModel.  Consumed by the upcoming
+    // GPU-compute cull (Phase 3E); the CPU cull still reads from bvh_items.
+    // Layout: struct { vec3 min; uint mesh_id; vec3 max; uint flags; } = 32 B.
+    // `flags` bit 0 = reflected (for winding-bucket selection).
+    GLuint aabb_ssbo = 0;
+    size_t aabb_ssbo_capacity = 0;  // bytes
+
     // Dynamic visible-instance index buffer (std430, binding = 1).
     // Re-uploaded each frame from visible_flat_.
     GLuint  visible_ssbo = 0;
@@ -215,6 +223,12 @@ private:
     bool growModelSsbo(ModelGpuData& m, size_t needed_total);
     ModelGpuData& getOrCreateModel(uint32_t model_id);
 
+    // (Re)build the per-instance world AABB SSBO from m.instances +
+    // m.instance_reflected.  One-shot upload called after finalizeModel /
+    // applyCachedModel once instances are settled.  Consumed by the GPU
+    // compute cull (Phase 3E, in progress).
+    void uploadInstanceAabbs(ModelGpuData& m);
+
     // Frustum-cull m's instances (BVH if available, else linear scan),
     // build the per-mesh DrawElementsIndirectCommand array + flat visible
     // list, and upload both to m.indirect_buffer / m.visible_ssbo.