ifcviewer: fix pick-pass cull corruption and cached-model ID collisions

Two stability bugs:

1. Clicking an object left the scene with wrong shading until the camera
   moved.  The pick pass re-culls every model with its own parameters
   (min_pixel_radius=0, no HiZ) and overwrites each model's visible_ssbo
   and indirect buffer.  The next render() saw an unchanged camera,
   skipped the cull via the have_cached_cull_ shortcut, and drew the
   stale pick-pass buffers.  Fix: invalidate have_cached_cull_ at the
   end of pickObjectAt().

2. Loading two sidecar-cached models made the second model's picked
   properties resolve to the first model's elements.  Sidecars store raw
   object_id / model_id values from the session that wrote them, and
   both files start at object_id=1, so element_map_ entries collided.
   Fix: on load, rebase every PackedElementInfo and InstanceCpu by
   (next_object_id_ - min_id_in_sidecar) and overwrite model_id with
   the freshly-assigned handle before the elements hit element_map_.

Also document both in the README — the pick-pass note under 3A
contribution culling, the sidecar rebase under the sidecar format
section.

src/ifcviewer/ViewportWindow.cpp

@@ -1282,6 +1282,13 @@ uint32_t ViewportWindow::pickObjectAt(int x, int y) {
 
     renderPickPass();
 
+    // The pick pass overwrote each model's visible_ssbo / indirect_buffer with
+    // pick-specific cull params (no contribution cull, no HiZ).  Invalidate
+    // the cached cull so the next render() rebuilds them with main-render
+    // params; otherwise the viewport draws with stale pick-pass buffers and
+    // shading looks wrong until the camera moves.
+    have_cached_cull_ = false;
+
     int px = x * devicePixelRatio();
     int py = (height() - y) * devicePixelRatio();
     uint32_t pixel = 0;