ifcviewer-web: multi-file loading (federation) via a per-model byte-source

The scene core is already multi-model — models_gpu_ is a map, applyCachedModel
APPENDS, and per-model model_id / object_id rebasing / georef+transformation
are how the desktop federates today. The only web-specific gap was the byte
source: web had ONE global source (__ifcvFile/__ifcvUrl) and reset the scene on
every load, so it could show one file at a time. Desktop meanwhile carries a
per-model source (streaming_file_path).

Mirror that on web: give each model its own web_source_id into a JS source
registry (Module.__ifcvSources[id] = a picked File or a sized remote URL).
beginWebChunkLoad, the metadata bootstrap, and the on-demand deferred fetch all
read from the owning model's source, so several files stream concurrently into
one federated scene — reusing all the shared machinery (viewAll, picking, the
GUID fetch) untouched.

- webReadRangesAsync / ifcvReadRangeInto / ifcvSourceSize take a source id.
- loadSidecarMetadataWeb(source_id, …) appends (no resetScene); main_web
  exposes load_sidecar_from_source_c(id) + clear_scene_c().
- URL size resolution moved to JS (shell.html registers + sizes sources via
  HEAD/Range), retiring the C-side ifcvBeginUrlSource / ifcv_source_ready dance.
- shell.html: source registry + "Open" (replace) / "Add" (append) buttons,
  multi-file selection; ?model= registers a URL source then loads.

Verified: two sidecars from two sources stream into one scene, both fully
resident, zero GPU errors. 111/111 unit + 6/6 web smoke pass.

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

src/ifcviewer-web/tests/smoke.spec.mjs

@@ -167,7 +167,7 @@ test('loads a user-picked sidecar through the Blob.slice byte-range path', async
   // side to confirm the blob load landed (logged to stderr → console).
   const samplePath = resolve(__dirname, '..', 'sample.ifcview');
   const loaded = page.waitForEvent('console', {
-    predicate: (m) => /loaded sidecar \(blob:/.test(m.text()),
+    predicate: (m) => /loaded sidecar \(source/.test(m.text()),
     timeout: 15_000,
   });
   await page.locator('#file-input').setInputFiles(samplePath);
@@ -208,7 +208,7 @@ test('streams a remote sidecar over HTTP Range (?model= URL backend)', async ({
 
   // Wait for the C side to confirm the URL-sourced load landed.
   const loaded = page.waitForEvent('console', {
-    predicate: (m) => /loaded sidecar \(net:/.test(m.text()),
+    predicate: (m) => /loaded sidecar \(source/.test(m.text()),
     timeout: 20_000,
   });
   await page.goto('/IfcViewerWeb.html?model=/sample.ifcview');