ifcviewer: visibility (hide/isolate/show-all) + X-ray — desktop parity on web

Lift the visibility + X-ray ops into ViewportCore so desktop and web share them.
The cull already reads visibility_ (hidden objects skipped) and the frame uniform
reads xray_alpha_cap_, both per frame, so each op just mutates state + schedules
a frame — no GPU buffers to rebuild, no rendering work:
- hideSelected (hide the selection, then deselect), isolateSelected (hide every
  non-selected object in a visible model), showAll (clear the hidden set),
  toggleXray (flip the alpha cap 1.0<->0.3; cull routes all to the transparent
  pass), xrayActive().
- Desktop ViewportWindow: the H / Shift+H / Alt+H / Alt+X keys and the menu-action
  wrappers now call the core; the four inline/duplicated implementations are gone.
- Web main_web: same keys (H hide · Shift+H isolate · Alt+H show all · Alt+X
  x-ray) + toolbar buttons (Hide / Isolate / Show all / X-ray, the last reflecting
  active state).

Verified on web: x-ray toggles and visibly translucent-izes the scene, hide after
a pick removes geometry, 0 GPU errors. 113/113 desktop + 6/6 web smoke; desktop
app builds.

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

src/ifcviewer/ViewportCore.h

@@ -624,6 +624,19 @@ public:
     // Marks selection_ dirty for the next render's flush.
     void applyPickToSelection(std::uint32_t object_id, bool add, bool remove);
 
+    // Visibility + X-ray, shared by desktop (H / Shift+H / Alt+H / Alt+X) and
+    // web. Hidden objects are skipped by the cull and xray_alpha_cap_ is read
+    // by the frame uniform, both per frame — so each call just mutates state and
+    // schedules a frame; no GPU buffers to rebuild.
+    void hideSelected();      // hide the selected objects, then clear selection
+    void isolateSelected();   // hide everything that is NOT selected
+    void showAll();           // clear the hidden set
+    size_t hiddenCount() const { return visibility_.hiddenCount(); }
+    // Global X-ray: translucent everything. Flips the frame uniform's alpha cap;
+    // the cull classifier routes every instance through the transparent pass.
+    void toggleXray();
+    bool xrayActive() const { return xray_alpha_cap_ < 1.0f; }
+
 #if defined(__EMSCRIPTEN__)
     // Async object pick for the web build: encodes the same pick pass as
     // pickObjectAt but reads the staging buffer back via a spontaneous map