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/InstancedGeometry.h

@@ -113,7 +113,7 @@ struct InstanceInfo {
     uint32_t mesh_id                  = 0;  // index into meshes array
     uint32_t object_id                = 0;
     uint32_t color_override_rgba8     = 0;
-    uint32_t model_id                 = 0;
+    uint32_t session_model_id                 = 0;
     double   placement_transformation[16]{};
     float    transform[16]{};
     float    world_aabb_min[3]{};
@@ -126,7 +126,7 @@ struct InstanceInfo {
 // geometry in local coords.  `local_mesh_id` is the streamer-assigned id
 // within this model.
 struct StreamedMesh {
-    uint32_t model_id      = 0;
+    uint32_t session_model_id      = 0;
     uint32_t local_mesh_id = 0;
     std::vector<float>    vertices;  // 7 floats * N_verts (pos3+norm3+color1_packed)
     std::vector<uint32_t> indices;
@@ -138,7 +138,7 @@ struct StreamedMesh {
 // iterator).  For the first instance of a mesh, the StreamedMesh is emitted
 // just before this.
 struct StreamedInstance {
-    uint32_t model_id             = 0;
+    uint32_t session_model_id             = 0;
     uint32_t local_mesh_id        = 0;
     uint32_t object_id            = 0;
     uint32_t color_override_rgba8 = 0;