ifcviewer: move edge silhouette subsystem into ViewportCore (#84-s)

buildEdgePipeline, encodeEdgePass, releaseEdgeResources + the EDGE_WGSL
shader source all move to ViewportCore. The edge_bind_group_ + the
edges_enabled_ flag come along too (the latter aliased on VW so the
edge-toggle keybind keeps compiling).

The pass binds the now-core-side depth_view_ directly, so there's no
remaining cross-side state dependency for edge rendering. render()
still calls core_.encodeEdgePass(enc, surface_view) — once render()
itself moves, the call collapses to a sibling method invocation.

src/ifcviewer/ViewportWindow.h

@@ -670,8 +670,8 @@ private:
     WGPUBindGroupLayout& edge_bgl_;
     WGPUPipelineLayout&  edge_pipeline_layout_;
     WGPURenderPipeline&  edge_pipeline_;
-    WGPUBindGroup       edge_bind_group_      = nullptr;
-    bool                edges_enabled_        = true;
+    WGPUBindGroup&      edge_bind_group_;
+    bool&               edges_enabled_;
 
     // Pivot visibility state — the gizmo itself lives in overlays_.
     // The timer auto-hides the pivot after a wheel-zoom afterglow.