ifcviewer-web: stream getObjects() per model instead of one scene-sized JSON

getObjects() is on every real host page's path — a click hands back an
object id, and resolving it to a GlobalId/name/type needs the element
tables (the JS layer also builds its GUID index from this call). The
implementation materialised a vector of ElementRef (three fresh
std::strings per element), serialised the entire scene into one JSON
string grown by +=, and UTF8ToString'd the whole thing — several
hundred MB simultaneously alive at ~600k elements. The wasm heap never
returns pages, so that transient became the session's permanent floor.

ViewportCore::visitModelElements hands out one model's elements as
slices into its string table (no per-element copies), and the export
serialises straight from those, one model per batch, reusing one string
whose capacity grows only to the largest model. The JS side accumulates
batches and resolves the same array as before — the page API is
unchanged. Peak is now one model's JSON instead of the scene's.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/ifcviewer/ViewportCore.cpp

@@ -4514,6 +4514,36 @@ std::vector<ViewportCore::ElementRef> ViewportCore::elements() const {
     return out;
 }
 
+void ViewportCore::visitModelElements(
+        int model_index,
+        const std::function<void(const ElementSlices&)>& visit) const {
+    const std::vector<std::uint32_t> ids = modelIdsInLoadOrder();
+    if (model_index < 0 || std::size_t(model_index) >= ids.size()) return;
+    auto it = models_gpu_.find(ids[std::size_t(model_index)]);
+    if (it == models_gpu_.end()) return;
+    const ModelGpuData& m = it->second;
+    auto slice = [&m](std::uint32_t offset, std::uint32_t length,
+                      const char*& out_ptr, std::uint32_t& out_len) {
+        // Out-of-range or zero-length offsets mean "no string was written".
+        if (length > 0 && std::size_t(offset) + length <= m.string_table.size()) {
+            out_ptr = m.string_table.data() + offset;
+            out_len = length;
+        } else {
+            out_ptr = nullptr;
+            out_len = 0;
+        }
+    };
+    for (const ElementTableRecord& e : m.elements) {
+        ElementSlices out;
+        out.object_id = e.object_id;
+        out.source_id = m.web_source_id;
+        slice(e.guid_offset, e.guid_length, out.guid, out.guid_len);
+        slice(e.name_offset, e.name_length, out.name, out.name_len);
+        slice(e.type_offset, e.type_length, out.type, out.type_len);
+        visit(out);
+    }
+}
+
 bool ViewportCore::elementForObject(std::uint32_t object_id, ElementRef& out) const {
     InstanceCompose::InstanceLookup lk;
     if (!findInstance(object_id, lk)) return false;