ifcviewer: overhaul model/object ID tracking

Rename the two overloaded model identifiers and make object_id
assignment single-authority, fixing a pick -> properties mismatch.

Identifiers:
- Per-model UUID fed_id -> model_id; the uint32 runtime handle
  model_id -> session_model_id (SessionState accessors + mirror hashes
  renamed to match). "fed_id" was a misnomer -- the federation is the
  whole collection, not one model.

object_id assignment (fixes wrong class on click):
- Producers (GeometryStreamer, .ifcview sidecar) now stamp model-LOCAL
  object_ids; ViewportCore::applyCachedModel is the sole authority that
  assigns the session-global id (base + local). Removed
  SceneLoader::next_object_id_, GeometryStreamer::lastObjectId(), and the
  streamer's start_object_id parameter.
- The element table is stamped by the same base on both load paths
  (applySidecarData and onStreamerFinished), so registry ids match the
  ids pick returns. Previously the sidecar path double-rebased instances
  vs the registry (click IfcSite -> showed IfcDoor); the live-stream path
  had the same latent mismatch. Both closed.

Naming / cleanup:
- SceneLoader::addFiles -> queueModels; startStreamLoadFor ->
  loadFromGeometryStreamer; readSidecarMetadataOnly -> readSidecarMetadata.
- Federation::addModel takes an explicit display_name (no QFileInfo
  fallback); callers pass QFileInfo(path).fileName().
- Disambiguate cryptic short locals (d->sidecar, m->model, c->chunk, ...)
  in SceneLoader, Federation, ViewportWindow, AreaMeasurement,
  SectionGizmoRenderer, and the SidecarData/SidecarReadPlan spots in
  ViewportCore.

Tests: 125/125 pass.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/ifcviewer/SidecarBuilder.cpp

@@ -103,7 +103,7 @@ void SidecarBuilder::onInstanceReady(const StreamedInstance& instance_record) {
     instance.mesh_id              = instance_record.local_mesh_id;
     instance.object_id            = instance_record.object_id;
     instance.color_override_rgba8 = instance_record.color_override_rgba8;
-    instance.model_id             = instance_record.model_id;
+    instance.session_model_id             = instance_record.session_model_id;
 
     // The streamer's instance transform is the double-precision
     // placement_transformation.  The cached float transform/world_aabb is only
@@ -142,7 +142,7 @@ SidecarData SidecarBuilder::finalize(const ModelGeoref& georef,
     for (const auto& info : elements) {
         ElementTableRecord packed;
         packed.object_id = info.object_id;
-        packed.model_id  = info.model_id;
+        packed.session_model_id  = info.session_model_id;
         packed.ifc_id    = info.ifc_id;
 
         packed.guid_offset = static_cast<uint32_t>(sidecar_data_.string_table.size());
@@ -191,8 +191,7 @@ bool SidecarBuilder::build(const QString& ifc_path,
     });
 
     streamer.loadFile(ifc_path.toStdString(),
-                      /*start_object_id*/ 1,
-                      /*model_id*/ 1,
+                      /*session_model_id*/ 1,
                       num_threads);
 
     loop.exec();