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/tests/test_sidecar_cache.cpp

@@ -87,7 +87,7 @@ SidecarData buildFixture() {
         inst.mesh_id   = (i < 3) ? 0u : 1u;
         inst.object_id = uint32_t(100 + i);
         inst.color_override_rgba8 = uint32_t(0xAA000000u | (i * 0x010203u));
-        inst.model_id  = 1;
+        inst.session_model_id  = 1;
         for (int k = 0; k < 16; ++k) {
             inst.placement_transformation[k] = double(i) * 0.25 + double(k);
             inst.transform[k]                = float(i) * 0.5f  + float(k);
@@ -111,7 +111,7 @@ SidecarData buildFixture() {
     for (size_t i = 0; i < sd.elements.size(); ++i) {
         ElementTableRecord& e = sd.elements[i];
         e.object_id = uint32_t(100 + i);
-        e.model_id  = 1;
+        e.session_model_id  = 1;
         e.ifc_id    = int32_t(1000 + i);
         e.guid_offset = 0;  e.guid_length = 0;
         e.name_offset = 1;  e.name_length = 4;   // "Wall"