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/SectionGizmoRenderer.cpp

@@ -313,16 +313,16 @@ void SectionGizmoRenderer::encode(WGPURenderPassEncoder pass, const Eigen::Matri
     const float vh     = float(viewport_h_px);
     const int   n      = std::min<int>(int(planes.size()), kMaxPlanes);
     for (int i = 0; i < n; ++i) {
-        const SectionPlane& p = planes[i];
+        const SectionPlane& plane = planes[i];
         Eigen::Vector3f nn, tangent, bitangent;
-        planeBasis(p.n, nn, tangent, bitangent);
+        planeBasis(plane.n, nn, tangent, bitangent);
         // Fixed 1 m gizmo (matches the desktop OverlayRenderer / GL constant).
         // NOT visual_radius: the normal is flipped toward the camera, so a large
         // arrow would shoot past the eye (clip.w<0) and vanish.
         const float half = 1.0f;
 
         uint8_t slot[256];
-        packSectionUniform(slot, view_proj, p.origin, half, tangent, line_w,
+        packSectionUniform(slot, view_proj, plane.origin, half, tangent, line_w,
                            bitangent, nn, 1.0f, 1.0f, 1.0f, 1.0f, vw, vh);
         const uint32_t slot_offset = uint32_t(i) * kSectionUniformSlot;
         wgpuQueueWriteBuffer(queue_, uniform_buffer_, slot_offset, slot, sizeof(slot));
@@ -340,12 +340,12 @@ int SectionGizmoRenderer::hitTest(int x, int y, const std::vector<SectionPlane>&
     float best_d = tolerance_px;
     const int n = std::min<int>(int(planes.size()), kMaxPlanes);
     for (int i = 0; i < n; ++i) {
-        const SectionPlane& p = planes[i];
+        const SectionPlane& plane = planes[i];
         // The arrow runs origin → origin + n * 1 m (visual radius scales the
         // gizmo, but hit-test the unit arrow to mirror the desktop).
         Eigen::Vector2f s_origin, s_tip;
-        if (!projectWorldToLogicalScreen(vp, p.origin, viewport_w_px, viewport_h_px, s_origin)) continue;
-        if (!projectWorldToLogicalScreen(vp, p.origin + p.n * 1.0f, viewport_w_px, viewport_h_px, s_tip)) continue;
+        if (!projectWorldToLogicalScreen(vp, plane.origin, viewport_w_px, viewport_h_px, s_origin)) continue;
+        if (!projectWorldToLogicalScreen(vp, plane.origin + plane.n * 1.0f, viewport_w_px, viewport_h_px, s_tip)) continue;
         const Eigen::Vector2f ab = s_tip - s_origin;
         const float ab_len2 = ab.squaredNorm();
         if (ab_len2 < 1e-3f) continue;