ifcviewer-web: contribution-cull streaming + combined loaded/needed/total bar

Streaming picked candidates on raw frustum visibility, so viewAll over a big
federation (all models in-frustum) fetched every chunk — even fine chunks that
project to sub-pixel and the renderer never draws. Gate streaming on the SAME
contribution decision the render path already makes.

- New per-chunk contribution_visible_count: instances that passed frustum AND
  the contribution cull (projected radius >= min_radius_px), counted BEFORE HiZ
  — so it's stable while the camera is still (unlike the HiZ-post counters,
  which flip frame-to-frame and would thrash the loader) and shifts only on
  navigation, when the working set should. The candidate gate skips chunks with
  count 0: the network pulls only what's resolvable now; the rest stream in as
  you approach. Verified: fit view needs the geometry, zoomed-out needs 0.

- Loading UI reworked from per-model segments to a combined bar over the whole
  federation: dark track = not needed for this view, dim = needed-but-unloaded,
  bright = loaded. "loaded / needed" = how done THIS view is; "needed / total" =
  how much of the model the view requires — "Loading 45 / 90 MB for this view ·
  12% of 718 MB total". Driven by ViewportCore::streamingByteProgress + ifcv_bytes_*.
  Two fixes from testing: (1) show a distinct overhead phase while total==0
  ("Loading model data — X MB · Y/N models ready") so the metadata download
  isn't a dead-looking bar; (2) re-assert display:block every active frame so the
  bar REAPPEARS when navigation reveals new chunks (was only set in
  beginLoadProgress → stayed hidden after the first catch-up). Per-model exports
  kept for a future detailed view.

111/111 unit + 6/6 web smoke pass.

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

src/ifcviewer/ViewportCore.h

@@ -416,6 +416,14 @@ public:
     void streamingModelProgress(int idx, int& resident_chunks,
                                 int& total_chunks) const;
 
+    // Byte-level streaming progress for a combined loading bar. total = all
+    // geometry bytes; needed = bytes the current view wants (contribution-
+    // culled working set); loaded = the resident subset of needed. Lets the UI
+    // show "loaded / needed" for this view and "needed / total" as context.
+    void streamingByteProgress(std::uint64_t& total_bytes,
+                               std::uint64_t& needed_bytes,
+                               std::uint64_t& loaded_bytes) const;
+
     // Direct-load (bonsai-side) entry points. Bonsai's SceneLoader feeds
     // the viewer one mesh + one instance at a time, then calls
     // finalizeModel once everything's staged. The staging map lives on