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-web/shell.html

@@ -88,35 +88,31 @@
     statusEl.classList.add('error');
   }
 
-  // File-browse loading. The picked file's bytes are written into MEMFS
-  // (the same virtual FS the wasm fopen()s) and then load_uploaded_model_c
-  // is invoked to read + render it. No drag-drop: that needs an X11 drag
-  // source (a file manager), which a minimal WM may not provide; the
-  // native file chooser this button opens is WM-independent.
+  // File-browse loading (#88, byte-range). The picked File object is stashed
+  // on Module.__ifcvFile and load_sidecar_from_blob_c reads it lazily via
+  // Blob.slice — the file is NOT copied into the wasm heap, so a 500 MB
+  // sidecar stays in the browser File object and only chunk-sized slices
+  // ever cross into wasm. No drag-drop: that needs an X11 drag source (a
+  // file manager), which a minimal WM may not provide; the native file
+  // chooser this button opens is WM-independent.
   var openBtn = document.getElementById('open-btn');
   var fileInput = document.getElementById('file-input');
   openBtn.addEventListener('click', function() { fileInput.click(); });
   fileInput.addEventListener('change', function(ev) {
     var f = ev.target.files && ev.target.files[0];
     if (!f) return;
-    if (!Module.FS || !Module._load_uploaded_model_c) {
+    if (!Module._load_sidecar_from_blob_c) {
       statusEl.textContent += 'viewer not ready yet — wait for WebGPU init\n';
       return;
     }
-    var reader = new FileReader();
-    reader.onload = function() {
-      try {
-        var bytes = new Uint8Array(reader.result);
-        try { Module.FS.mkdir('/uploads'); } catch (e) { /* already exists */ }
-        Module.FS.writeFile('/uploads/model.ifcview', bytes);
-        Module._load_uploaded_model_c();
-      } catch (e) {
-        statusEl.textContent += 'load failed: ' + e + '\n';
-        statusEl.classList.add('error');
-      }
-      fileInput.value = '';  // let the same file be re-picked
-    };
-    reader.readAsArrayBuffer(f);
+    try {
+      Module.__ifcvFile = f;          // kept alive for lazy Blob.slice reads
+      Module._load_sidecar_from_blob_c();
+    } catch (e) {
+      statusEl.textContent += 'load failed: ' + e + '\n';
+      statusEl.classList.add('error');
+    }
+    fileInput.value = '';  // let the same file be re-picked
   });
 </script>
 <!-- IfcViewerWeb.js is emitted alongside this shell by the emcc build;