ifcviewer: v15 — defer property metadata off the first-paint path

First-paint over a network is metadata-bound: the whole post-index metadata
(~10 MB on a 118 MB model) had to download before any geometry. But ~25% of
it — elements + string_table, the IFC element tree (names/GUIDs/hierarchy) —
is used only for UI/picking, never for rendering (ViewportCore never touches
it).

v15 splits the post-index metadata into a render-CRITICAL block (meshes,
instances, georef, chunk TOC) preceded by its byte length, then a DEFERRED
block (elements + string_table). The web loader reads only the critical block
before painting; the deferred block sits at a known, self-describing offset
([critical end, EOF)) and is fetched on demand. Desktop reads both (local).

Web on-demand path is wired and complete (not yet called — no UI consumer):
loadDeferredMetadataWeb(model_id) range-fetches + parses the deferred block
into ModelGpuData.elements/string_table, at most once; the first consumer
will be "show the selected object's name" on pick. No background prefetch —
view-only sessions never download the property data (saves 2.64 MB on this
model).

parseSidecarTail split into parseSidecarCritical + parseSidecarDeferred (pure,
unit-tested); StreamingSidecar gains the critical-block locator. Measured
(118 MB model): critical metadata 10.35 -> 7.71 MB, deferred 2.64 MB off the
path; first paint 10.5 -> 9.6 s @ 24 Mbps. (Instances still dominate the
critical block — the next metadata lever.) Format -> v15, no back-compat;
regenerate sidecars. 111/111 unit + 6/6 web smoke pass.

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

src/ifcviewer/ViewportCore.h

@@ -371,6 +371,14 @@ public:
     void loadSidecarFromUrlWeb(std::string url);
     void loadSidecarMetadataWeb(std::string source_label);
 
+    // On-demand fetch of the v15 deferred property block (element tree + string
+    // 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.
+    void loadDeferredMetadataWeb(std::uint32_t model_id,
+                                 std::function<void(bool)> done = {});
+
     // 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