ifcviewer: edge enhancement post-pass

Adds a per-frame depth-laplacian pass that darkens pixels at sharp
depth discontinuities — silhouettes, overlapping-surface boundaries,
section-cut edges.  Catches the wall-against-wall and slab-against-
ceiling cases that the cavity hint in the lighting shader misses.

Implementation:

- New edge_depth_fbo_ / edge_depth_tex_ — single-sample D24S8 the
  size of the window.  After the main draw, blit the default FB
  depth into it (handles MSAA resolve in the same call).
- Fullscreen triangle generated from gl_VertexID, samples four
  cardinal neighbours, computes |4c - n - s - e - w| on linearized
  depth.  Linearization branches between perspective and ortho via
  u_is_ortho.  Threshold scales with depth so distant edges still
  register.
- Output is multiplicatively blended (GL_DST_COLOR, GL_ZERO) so
  colours just darken; no separate composite step.
- Runs before the pivot/section/axis gizmos so they aren't outlined
  themselves.  HiZ pyramid build still runs after, unchanged.

Per-frame cost is one MSAA depth blit + one fullscreen pass with
five depth samples.  Sub-millisecond at 1080p on a mid GPU.

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

src/ifcviewer/ViewportWindow.h

@@ -289,6 +289,11 @@ private:
     void renderPivotIndicator();
     void renderSectionPlanes();
     void buildSectionPlaneGizmo();
+    // Post-process edge enhancement: resolve MSAA depth into a single-
+    // sample texture, then run a fullscreen pass that detects sharp
+    // depth-laplacian peaks and darkens the colour buffer there.  Catches
+    // silhouettes and overlapping-surface boundaries as faint dark lines.
+    void renderEdgePass();
     // Returns the index of the section plane whose arrow gizmo is under
     // (x, y), or -1 if none.  Screen-space line-segment distance test.
     int  hitTestSectionGizmo(int x, int y) const;
@@ -548,6 +553,16 @@ private:
     int    plane_arrow_offset_ = 0;
     int    plane_arrow_count_  = 0;
 
+    // Edge-enhancement pass resources.  edge_depth_tex_ is a single-sample
+    // resolve target the size of the window; we blit the default FB depth
+    // into it each frame, then sample it from the fullscreen edge shader.
+    GLuint edge_program_    = 0;
+    GLuint edge_depth_fbo_  = 0;
+    GLuint edge_depth_tex_  = 0;
+    GLuint edge_vao_        = 0;   // empty VAO for fullscreen-triangle draw
+    int    edge_w_ = 0;
+    int    edge_h_ = 0;
+
     // FPS smoothing
     int frame_count_ = 0;
     float accumulated_time_ = 0.0f;