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/ViewportCore.h

@@ -375,10 +375,16 @@ public:
     // table) for a web-streamed model — what a UI (object tree / selected-name
     // / search) needs, fetched only when asked so first paint never waits on
     // it. Populates ModelGpuData.elements/string_table; fires done(ok). At most
-    // one fetch per model. Currently unwired (no consumer yet) but complete.
+    // one fetch per model.
     void loadDeferredMetadataWeb(std::uint32_t model_id,
                                  std::function<void(bool)> done = {});
 
+    // Demo consumer of the deferred fetch: on pick, ensure the owning model's
+    // property block is loaded (loadDeferredMetadataWeb — fetched once, on
+    // demand), then log the picked object's IFC GUID. The first pick triggers
+    // the network fetch; later picks reuse the cached element table.
+    void logSelectedObjectGuidWeb(std::uint32_t object_id);
+
     // Kick off the async read of one chunk's vertex + index byte ranges (from
     // the active web source). applyStreamedChunk runs in the JS completion
     // callback; c.is_loading is held until then. No-op if the model/chunk