ifcviewer-web: stream user sidecars via Blob.slice byte ranges (#88)

Picked files are no longer copied whole into the wasm heap. The browser
File object stays in JS (Module.__ifcvFile) and is read lazily through
Blob.slice byte ranges, so a 200-500 MB sidecar never enters wasm linear
memory — only chunk-sized slices do.

Mechanism (web-only, #if __EMSCRIPTEN__):

  - JS glue (EM_JS): ifcvFileSize + ifcvReadRangeInto — slice [off,off+n)
    of the File and copy it into a caller-provided heap pointer, then call
    back _ifcv_on_range_done. No malloc across the boundary; C pre-sizes
    the destination from the read plan.
  - webReadRangesAsync: reuses planSidecarReadRanges to coalesce a range
    set into Blob.slice reads (1 MB gap — each slice is an async hop),
    scatters them into a destination laid out in input order, and fires a
    continuation when the whole set lands. An in-flight map keyed by id
    survives unordered_map rehash (scratch buffers are heap-owned).
  - loadSidecarFromBlobWeb: async metadata load — head (16 B) -> index
    count -> tail-to-EOF -> parseSidecarHead/Tail -> applyCachedModel, then
    tags the model streaming_from_blob and frames it.
  - driveStreamingLoads: blob-sourced models route to beginWebChunkLoad
    (async vertex+index range reads -> applyStreamedChunk in the callback),
    holding is_loading until the bytes arrive. The embedded MEMFS sample
    keeps the synchronous fopen path.

shell.html stashes the File and calls _load_sidecar_from_blob_c instead of
FS.writeFile'ing the whole thing; EXPORTED_RUNTIME_METHODS=['FS'] dropped.
Desktop is untouched (the new members + driveStreamingLoads branch are all
emscripten-guarded). Web links clean; desktop rebuilds; 107/107 unit tests
pass.

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

src/ifcviewer/ModelGpuData.h

@@ -277,6 +277,11 @@ struct ModelGpuData {
     std::string streaming_file_path;
     uint64_t    streaming_vertex_section_offset = 0;
     uint64_t    streaming_index_section_offset  = 0;
+    // Web only: chunk byte ranges come from the JS-registered File via
+    // Blob.slice (async), not from a synchronous fopen on streaming_file_path.
+    // Set by loadSidecarFromBlobWeb so driveStreamingLoads routes this model
+    // through the async blob path instead of the MEMFS sync read.
+    bool        streaming_from_blob = 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