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/StreamingThread.h

@@ -44,7 +44,7 @@
 class StreamingThread {
 public:
     struct Request {
-        uint32_t     model_id;
+        uint32_t     session_model_id;
         std::size_t  chunk_idx;
         std::string  file_path;
         // v16: the chunk's two zstd frames in the geometry section. The reader
@@ -56,7 +56,7 @@ public:
     };
 
     struct Result {
-        uint32_t              model_id;
+        uint32_t              session_model_id;
         std::size_t           chunk_idx;
         bool                  success;
         std::vector<uint8_t>  vbytes;