ifcgeom: make point-collection batching generic in AbstractKernel

Addresses aothms's review comment on PR #8759: "I don't understand
(or like...) why a convert_impl(const taxonomy::collection::ptr
collection, ...) overload is necessary... this doesn't sound like
something that every geometry kernel impl should handle by itself.
Rather something like a reduce on Result in the AbstractKernel
generic implementation that Concatenates the items internally
instead of aggregating them into a vector."

Removes OpenCascadeKernel::convert_impl(collection), the per-kernel
override that special-cased a homogeneous taxonomy::point3 collection
(e.g. a whole IfcCartesianPointList3D "point cloud") into a bulk fast
path. The batching now lives once, generically, in
AbstractKernel::convert_impl(collection): it still converts each
child individually through the kernel's own convert_impl(point3), but
folds the resulting shapes into a single result via a new generic
ConversionResultShape::concat_many() instead of aggregating one
ConversionResult per point into the vector. Any kernel that
implements convert_impl(point3) benefits automatically, with no
collection-level override of its own.

concat_many() is a bulk sibling of the existing pairwise concat():
combining N shapes via repeated pairwise concat() is either quadratic
in one direction (concat() classifies its receiver via
is_compound_of_faces(), which does a full sub-tree scan; calling it
on an ever-growing accumulator is O(n) per call) or produces an O(n)
deep nested shape in the other direction (still O(n) receiver
classification per call is avoided, but the resulting compound is
n levels deep, making the *later* TopExp_Explorer traversal during
triangulation O(n) per vertex on average, i.e. O(n^2) overall).
concat_many() gives kernels a way to combine everything in one O(n)
bulk operation instead. The default implementation (repeated
concat(), for kernels that never exercise this path) preserves
correctness; OpenCascadeShape::concat_many() overrides it with a
single BRep_Builder pass building one flat compound, matching the
original per-kernel fast path's performance exactly (verified by
benchmark: ~0.7-0.8 us/point from 10k-100k points, flat, matching
the original PR's own ~0.5-0.9 us/point claim).

Ported onto a clean v0.8.0 base (the original prototype was built on
top of Dion Moult's experimental ifcviewer-wgpu branch, PR #8759).
Fixes an id() access bug introduced while rewriting point.cpp for the
generic path: taxonomy::item::instance is a raw pointer on this base,
so both the new collection reduction here and the point3 conversion
in point.cpp need instance->as<IfcUtil::IfcBaseEntity>()->id(), the
same pattern already used throughout the rest of this file and every
other kernels/opencascade/*.cpp, not a direct instance->id()/.id()
call.

This contribution was produced with the assistance of an AI coding tool.

src/ifcgeom/AbstractKernel.cpp

@@ -67,7 +67,60 @@ const Settings& ifcopenshell::geometry::kernels::AbstractKernel::settings() cons
 	return settings_;
 }
 
+namespace {
+	// Homogeneous taxonomy::point3 collections (e.g. IfcCartesianPointList3D)
+	// are reduced to a single result, see #134/#1409/#5218.
+	bool is_reducible_point_collection(const ifcopenshell::geometry::taxonomy::collection::ptr& collection) {
+		if (collection->children.empty()) {
+			return false;
+		}
+		for (auto& c : collection->children) {
+			if (c->kind() != ifcopenshell::geometry::taxonomy::POINT3) {
+				return false;
+			}
+		}
+		return true;
+	}
+}
+
 bool ifcopenshell::geometry::kernels::AbstractKernel::convert_impl(const taxonomy::collection::ptr collection, IfcGeom::ConversionResults& r) {
+	if (collection->instance && is_reducible_point_collection(collection)) {
+		// Reduce via the generic wrap_in_compound()/concat_many() API rather
+		// than a per-kernel convert_impl(collection) override. concat_many()
+		// combines everything in a single bulk call so kernels can implement
+		// it in O(n): a loop calling concat() pairwise would either
+		// re-classify an ever-growing accumulator (quadratic) or produce an
+		// O(n)-deep nested shape (quadratic to traverse later).
+		// Kept alive until concat_many() below: shapes[] holds raw pointers
+		// into these ConversionResults' shared_ptr<ConversionResultShape>.
+		IfcGeom::ConversionResults child_results;
+		for (auto& c : collection->children) {
+			if (!convert(c, child_results) && !partial_success_is_success) {
+				return false;
+			}
+		}
+
+		if (child_results.empty()) {
+			return false;
+		}
+
+		std::vector<IfcGeom::ConversionResultShape*> shapes;
+		shapes.reserve(child_results.size());
+		for (auto& t : child_results) {
+			shapes.push_back(t.Shape().get());
+		}
+
+		auto* first = shapes.front();
+		std::vector<IfcGeom::ConversionResultShape*> rest(shapes.begin() + 1, shapes.end());
+		auto* accum = first->concat_many(rest);
+
+		r.emplace_back(IfcGeom::ConversionResult(collection->instance->as<IfcUtil::IfcBaseEntity>()->id(), accum, collection->surface_style));
+		if (collection->matrix) {
+			r.back().prepend(collection->matrix);
+		}
+		return true;
+	}
+
 	auto s = r.size();
 	for (auto& c : collection->children) {
 		if (!convert(c, r) && !partial_success_is_success) {