ifcviewer: don't block the UI while baking the .ifcview at 100%

Opening a fresh .ifc streams geometry to the GPU, then bakes the .ifcview
cache. That bake — reorder + per-chunk zstd (level 19) — ran synchronously
in SceneLoader::onStreamerFinished, which is a QueuedConnection slot on the
main thread, so it froze the UI right as the progress bar hit 100% (≈15s of
zstd for a 130 MB-geometry model).

- Move the compress + writeSidecar onto a background thread. The geometry is
  already resident and the sidecar is only a cache for the next open, so the
  viewport is interactive the instant streaming finishes; the write is joined
  before the next write and in the destructor.
- Parallelise the per-chunk zstd across hardware_concurrency threads (compress
  all chunks, then write serially to keep contiguous offsets) so the
  background write also finishes quickly.

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

src/ifcviewer/SceneLoader.h

@@ -173,6 +173,10 @@ private:
     uint32_t next_session_model_id_ = 1;
     uint32_t loading_session_model_id_ = 0;
     std::thread sidecar_read_thread_;
+    // Background .ifcview compress + write, so the seconds of zstd on a big
+    // model don't freeze the UI at 100%. Joined before the next write and in
+    // the destructor so a pending write always completes.
+    std::thread sidecar_write_thread_;
     // One thread per sidecar-hit model while its .rdb/.ifc opens in the
     // background.  Joined only at destruction so a slow SPF parse on model
     // A never blocks the sidecar-hit path of model B.