wgpu: overlay-line groups (stroke + dash, per-group dynamic uniform offset)

Ports GL OverlayRenderer's LineGroup API to WgpuOverlayRenderer. Each
group's segments are CPU-expanded into screen-space quads; the WGSL
fragment reproduces the GL pixel-distance stroke pick + arc-length
dash logic. One uniform slot per group, bound via dynamic offset so a
single bind-group services up to N groups.

No caller yet — sets up the API the wgpu measure tools (task #29) will
use. WgpuViewportWindow.setOverlayLines mirrors the GL viewport's
signature so the bonsai Measurement code can target either backend
through one interface.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/ifcviewer-wgpu/WgpuViewportWindow.h

@@ -281,6 +281,12 @@ private:
     void  removeSectionPlane(int index);
     void  clearSectionPlanes();
     int   sectionPlaneCount() const { return int(section_planes_.size()); }
+
+    // Overlay primitives. Mirror GL ViewportWindow so the Measurement +
+    // dimension tools can target either backend through one API.
+    // Empty groups clears the current set.
+    void  setOverlayLines(const std::vector<WgpuOverlayRenderer::LineGroup>& groups);
+
     void  ensureHizTextures(int viewport_w, int viewport_h);
     void  releaseHizResources();
     // Resolves the just-rendered MSAA depth into the small single-sample