ifcviewer: v16 zstd-compressed sidecars (~10x smaller over the wire)

The .ifcview data is hugely redundant (repeated double instance matrices,
patterned indices) — measured 12x zstd whole-file. Server Content-Encoding
can't be used (it breaks HTTP Range), so compress PER-CHUNK into the format.

Format (v16): geometry becomes per-chunk zstd(vertices)+zstd(indices) frames —
each independently Range-fetchable, so streaming is intact — and the critical +
deferred metadata blocks are single zstd frames. SidecarChunk carries the
compressed blob offsets/sizes; applyStreamedChunk (render/upload) is UNCHANGED —
decompression slots into the fetch. Full readSidecar (test/tooling) reconstructs
by decompress+scatter. zstd: desktop links libzstd (also compresses at bake);
the web build (Emscripten has no zstd port) FetchContent's the pinned zstd
source and compiles its decompress-only subset for wasm — no vendored blob,
same version as desktop. New SidecarCompress wraps it (compress guarded off
under Emscripten). Both stream paths — desktop StreamingThread worker + sync
fallback (readChunkGeometryCompressed) and web beginWebChunkLoad — decompress;
readSidecarMetadataOnly / the web bootstrap / loadDeferredMetadataWeb decompress
the metadata blocks. streamingByteProgress reports COMPRESSED bytes. MEASURED: a
752 MB v15 federation → 75 MB v16 (10x; per-file 6.7-15.3x); PP-PLP 118→15 MB,
loads 13/13 chunks on web, 0 errors.

Three fixes found while testing big federations on a real server:
- Web-streamed race: streaming_from_web was set in the deferred-header callback
  (a round-trip after the model+chunks exist), so driveStreamingLoads could take
  the sync fopen path meanwhile → "failed to read/decompress chunk 0". Now set
  immediately after applyCachedModel.
- OOM abort on 18 models: the pool grew unbounded until an alloc failed, but on
  web that's an uncatchable bad_alloc abort. Cap total pool capacity
  (setMaxTotalCapacity, 3 GB) so it stops before the heap ceiling, and raise
  MAXIMUM_MEMORY 2→4 GB (wasm32 max) for headroom.
- Web never evicted (grow-or-block only). At the hard budget, fall through to the
  LRU/priority evictor so a big federation stays navigable (highest-contribution
  chunks win) instead of freezing with holes.

113/113 desktop + 6/6 web smoke pass. No back-compat: regenerate sidecars
(desktop bakes v16; scratch conv tool migrates v15→v16).

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

src/ifcviewer/StreamingThread.h

@@ -44,15 +44,15 @@
 class StreamingThread {
 public:
     struct Request {
-        uint32_t                                       model_id;
-        std::size_t                                    chunk_idx;
-        std::string                                    file_path;
-        uint64_t                                       vertex_section_offset;
-        uint64_t                                       index_section_offset;
-        // (section-relative byte_offset, byte_size)
-        std::vector<std::pair<uint64_t, uint64_t>>     v_ranges;
-        // (first_u32, count_u32)
-        std::vector<std::pair<uint64_t, uint64_t>>     i_ranges;
+        uint32_t     model_id;
+        std::size_t  chunk_idx;
+        std::string  file_path;
+        // v16: the chunk's two zstd frames in the geometry section. The reader
+        // fetches [geometry_section_offset + *_comp_off, +*_comp_size) and
+        // decompresses to *_raw_size.
+        uint64_t     geometry_section_offset = 0;
+        uint64_t     v_comp_off = 0, v_comp_size = 0, v_raw_size = 0;
+        uint64_t     i_comp_off = 0, i_comp_size = 0, i_raw_size = 0;
     };
 
     struct Result {