ifcviewer-web: stream remote sidecars over HTTP Range (?model=URL)

Adds a network byte-source alongside the local Blob one. The async-chunk
infra is source-agnostic — only the two JS primitives knew it was a Blob —
so this generalises them and reuses everything else:

  - ifcvReadRangeInto: local → Blob.slice; remote → fetch() with a Range
    header (206). If a server ignores Range and returns 200, the requested
    window is sliced out so it still works (without the bandwidth saving).
  - ifcvFileSize: Blob size, or the URL's total length resolved up front.
  - ifcvBeginUrlSource: resolves total size (HEAD Content-Length, else a
    0-0 ranged GET's Content-Range) then fires _ifcv_source_ready.
  - The metadata bootstrap is extracted into a source-agnostic
    loadSidecarMetadataWeb(label); loadSidecarFromBlobWeb / FromUrlWeb are
    thin entries. streaming_from_blob → streaming_from_web (now covers both).

main_web exports load_sidecar_from_url_c(url); shell.html reads a
?model=URL query param and ccalls it once the app is live (same-origin
needs no CORS; cross-origin hosts must send CORS + Accept-Ranges).

Test: serve.mjs now answers HEAD + Range (206) and falls back to the
ifcviewer-web source dir for sample.ifcview (embedded in the wasm, not in
build-web). New smoke case loads ?model=/sample.ifcview and asserts it
renders via the Range path. 6/6 web smoke + 107/107 unit pass; desktop
unaffected (web-guarded; only the shared field rename touches it).

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

src/ifcviewer-web/main_web.cpp

@@ -205,6 +205,17 @@ extern "C" EMSCRIPTEN_KEEPALIVE void load_sidecar_from_blob_c() {
     g_app->core.loadSidecarFromBlobWeb();
 }
 
+// Called from shell.html (e.g. a ?model=URL query param) to stream a sidecar
+// hosted at `url` via HTTP Range requests — the same per-chunk byte-range path
+// as the local File load, but the bytes come off the network instead of a
+// Blob. Asynchronous; the model frames itself once metadata lands. Exported
+// to JS via EXPORTED_FUNCTIONS in CMakeLists.txt.
+extern "C" EMSCRIPTEN_KEEPALIVE void load_sidecar_from_url_c(const char* url) {
+    if (!g_app || !g_app->ready || !url) return;
+    g_app->core.resetScene();
+    g_app->core.loadSidecarFromUrlWeb(url);
+}
+
 int main(int /*argc*/, char** /*argv*/) {
     Log::info() << "ifcviewer-web: starting";
     g_app = new AppState();