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/ModelGpuData.h

@@ -34,6 +34,7 @@
 #include "InstancedGeometry.h"
 #include "BufferPool.h"
 #include "ChunkPlanner.h"  // WGPU_CHUNK_VERTEX_BYTES_LIMIT (shared with bake)
+#include "SidecarCache.h"  // PackedElementInfo (deferred property metadata)
 
 // Per-model wgpu state. Mirrors the GL backend's ModelGpuData but with
 // wgpu handles. Stage 2 only allocates and uploads the four core buffers;
@@ -287,6 +288,18 @@ struct ModelGpuData {
     // instead of the MEMFS sync read.
     bool        streaming_from_web = false;
 
+    // v15 deferred property metadata (web, on-demand). The IFC element tree
+    // (elements + string_table — names/GUIDs/hierarchy, for UI/picking, never
+    // rendering) lives in a separate file block fetched only when a consumer
+    // asks, so first paint doesn't wait on it. Empty until
+    // loadDeferredMetadataWeb fetches [deferred_meta_offset, +bytes) and parses
+    // it; deferred_meta_loaded latches so it fetches at most once.
+    std::vector<PackedElementInfo> elements;
+    std::string                    string_table;
+    uint64_t    deferred_meta_offset = 0;
+    uint64_t    deferred_meta_bytes  = 0;
+    bool        deferred_meta_loaded = false;
+
     // 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
     // at applyCachedModel time; consumed by cullModelCpuCompute when it