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/CMakeLists.txt

@@ -143,10 +143,46 @@ set(IFCVIEWER_CORE_SOURCES
     InstanceCompose.cpp
     LodBuilder.cpp
     SidecarCache.cpp
+    SidecarCompress.cpp
     StreamingLoader.cpp
     StreamingThread.cpp
     ViewportCore.cpp
 )
+# Web needs a zstd DECODER (Emscripten has no zstd port; the desktop links the
+# full libzstd below). Rather than vendor a generated blob, fetch the pinned
+# zstd source and compile its decompress-only subset — the exact set zstd's
+# own single-file decoder inlines — straight into the wasm.
+if(EMSCRIPTEN)
+    include(FetchContent)
+    FetchContent_Declare(zstd_dec
+        GIT_REPOSITORY https://github.com/facebook/zstd.git
+        GIT_TAG        v1.5.7
+        GIT_SHALLOW    TRUE
+        # zstd's CMake lives in build/cmake, not the root — point SOURCE_SUBDIR
+        # at a non-existent dir so MakeAvailable only POPULATES the source and
+        # never tries to configure zstd's own (non-emscripten) build.
+        SOURCE_SUBDIR  does-not-exist)
+    FetchContent_MakeAvailable(zstd_dec)
+    set(ZSTD_DEC_DIR "${zstd_dec_SOURCE_DIR}/lib")
+    set(ZSTD_DEC_SRC
+        ${ZSTD_DEC_DIR}/decompress/zstd_decompress.c
+        ${ZSTD_DEC_DIR}/decompress/zstd_decompress_block.c
+        ${ZSTD_DEC_DIR}/decompress/zstd_ddict.c
+        ${ZSTD_DEC_DIR}/decompress/huf_decompress.c
+        ${ZSTD_DEC_DIR}/common/entropy_common.c
+        ${ZSTD_DEC_DIR}/common/error_private.c
+        ${ZSTD_DEC_DIR}/common/fse_decompress.c
+        ${ZSTD_DEC_DIR}/common/pool.c
+        ${ZSTD_DEC_DIR}/common/threading.c
+        ${ZSTD_DEC_DIR}/common/xxhash.c
+        ${ZSTD_DEC_DIR}/common/zstd_common.c
+        ${ZSTD_DEC_DIR}/common/debug.c)
+    # wasm isn't x86-64 → no BMI2 asm path (belt-and-braces disable).
+    set_source_files_properties(${ZSTD_DEC_SRC} PROPERTIES
+        COMPILE_DEFINITIONS "ZSTD_DISABLE_ASM=1")
+    # NOT appended to IFCVIEWER_CORE_SOURCES — those get a source-dir PREPEND
+    # below which would mangle these absolute paths; added via target_sources.
+endif()
 set(IFCVIEWER_CORE_HEADERS
     BufferPool.h
     CameraMath.h
@@ -162,6 +198,7 @@ set(IFCVIEWER_CORE_HEADERS
     SectionPlane.h
     SelectionState.h
     SidecarCache.h
+    SidecarCompress.h
     StreamingLoader.h
     StreamingThread.h
     VertexQuantization.h
@@ -183,6 +220,19 @@ if(UNIX AND NOT APPLE)
     find_package(Threads REQUIRED)
     target_link_libraries(IfcViewerCore PUBLIC Threads::Threads)
 endif()
+# Sidecar (de)compression. Web compiles the vendored single-file decoder (added
+# to the sources above) + uses the vendored zstd.h; desktop links the full
+# libzstd (compress + decompress) and takes zstd.h from the system.
+if(EMSCRIPTEN)
+    target_sources(IfcViewerCore PRIVATE ${ZSTD_DEC_SRC})
+    target_include_directories(IfcViewerCore PRIVATE ${ZSTD_DEC_DIR})
+else()
+    find_library(ZSTD_LIBRARY NAMES zstd libzstd)
+    if(NOT ZSTD_LIBRARY)
+        message(FATAL_ERROR "libzstd not found (needed for .ifcview compression)")
+    endif()
+    target_link_libraries(IfcViewerCore PUBLIC ${ZSTD_LIBRARY})
+endif()
 install(TARGETS IfcViewerCore EXPORT ${IFCOPENSHELL_EXPORT_TARGETS})
 
 # IfcViewer: the Qt + IfcGeom + OpenCASCADE shell — everything in this