ifcviewer-web: pick logs the object's IFC GUID via the on-demand deferred fetch

First real consumer of the v15 deferred property block, and an end-to-end
demonstration that on-demand property loading works. On a left-click pick,
logSelectedObjectGuidWeb ensures the owning model's deferred block is loaded
(loadDeferredMetadataWeb — a network fetch the FIRST time, cached after) and
logs the picked object's GUID to the console.

Fix uncovered while wiring it: applyCachedModel rebases instance object_ids to
a per-model global base (object_id_base) to keep them unique across models,
but the deferred elements carry the sidecar's original local ids — so a lookup
by the picked (global) id missed. Store object_id_base on the model and rebase
the elements by it when the deferred block loads.

Verified: with a streamed model, the deferred block is fetched ONLY after the
first pick (not at load), and the pick logs a valid 22-char IFC GUID. 111/111
unit + 6/6 web smoke pass.

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

src/ifcviewer/ModelGpuData.h

@@ -299,6 +299,10 @@ struct ModelGpuData {
     uint64_t    deferred_meta_offset = 0;
     uint64_t    deferred_meta_bytes  = 0;
     bool        deferred_meta_loaded = false;
+    // applyCachedModel rebases instance object_ids by this base to keep them
+    // globally unique across models; deferred elements carry the sidecar's
+    // original (local) ids, so they're rebased by the same amount on load.
+    uint32_t    object_id_base = 0;
 
     // For each mesh in meshes[], the chunk it lives in plus the chunk-local
     // offsets into that chunk's vertex_storage and index_buffer. Populated