web: MODULARIZE build + embedded JS-integration example + selection callback

Restructure the web viewer so the wasm is a reusable module and add a
second example that drives it from ordinary page DOM.

Build:
- Emit IfcViewerWeb.js (a `createIfcViewer` factory, MODULARIZE) + .wasm
  instead of a single baked page (dropped --shell-file); copy the static
  example pages next to it at build time.
- Unbreak the web build: CameraMath.h / ViewportCore.cpp used
  boost::math::constants::pi just for pi, pulling all of boost/math into a
  header shared with the Emscripten build (no Boost in its sysroot). Replace
  with a constexpr kPiF — identical value, no dependency, desktop unaffected.

JS integration (web/ifcviewer.js):
- A small helper wraps the factory: boots the viewer on a canvas, runs the
  RAF loop from onRuntimeInitialized (NOT a post-await .then, which stalls
  Dawn-web's device callback and leaves the device half-initialised), and
  exposes addFile/addUrl, clearScene, model list/progress, and onSelect(...).
- ViewportCore/main_web emit each pick to JS via Module.__ifcvOnSelect
  (object id + IFC GlobalId + model index; empty on deselect); onSelect also
  dispatches an 'ifcviewer:select' DOM event.
- Fix input coords for a non-fullscreen canvas: mousemove/mouseup are
  window-targeted, so convert their coords to canvas-relative via the canvas
  client-rect origin (marquee + box-pick were offset when embedded).

Examples:
- IfcViewerWeb.html: the fullscreen viewer (same DOM/behaviour as before,
  now loading the module) — the Playwright smoke suite still targets it.
- embedded.html: a sized viewer with DOM outside it to add models (file or
  URL), list loaded models with streaming progress, and show the model +
  GlobalId of the clicked object. Starts empty (drops the wasm's embedded
  sample, which the fullscreen page/tests still use).
- index.html links both.

Federation note: the web viewer already streams multiple models into one
scene (a byte-source per file/URL); it doesn't need the desktop Federation
document for this. Verified: 11/11 web smoke tests pass; embedded example
loads models, reports the picked model + GUID, and the marquee aligns.

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

src/ifcviewer-web/WebViewportHost.h

@@ -35,7 +35,7 @@
 class WebViewportHost final : public ViewportHost {
 public:
     // `canvas_selector` is the CSS selector for the host <canvas> (e.g.
-    // "#viewer-canvas" — matches shell.html). The string is stored;
+    // "#viewer-canvas" — matches the host page (web/ifcviewer.js)). The string is stored;
     // it must outlive the host.
     explicit WebViewportHost(std::string canvas_selector);