ifcviewer-full: length tool (2/3/4+ point distance, angle, polygon area)

ViewportWindow trades the area_tool_active_ bool for an enum ToolMode
{None, Area, Length}; the existing surfacePickedInTool signal carries
both, the app dispatches on toolMode().  Esc exits any active tool;
Backspace/Delete in length mode emits toolBackspacePressed which the
length tool uses to remove the last point.

LengthMeasurement collects clicked world-space points and adapts the
readout: 2pt → distance + axis-aligned ΔX/ΔY/ΔZ, 3pt → angle at the
middle vertex + triangle area, 4+pt → best-fit-plane PCA + shoelace
when planar (RMS plane distance / bbox diag < 1e-3) else fan
triangulation, with the chosen method labelled in the readout.  Per-
segment lengths float at each midpoint.

OverlayRenderer grows three new pipelines to support this:
  - point sprite shader: gl_PointCoord-based outlined disc with
    fwidth-smoothed inner/stroke bands, a single draw call.
  - line shader: CPU-expand each segment to 6 verts carrying both
    endpoints + (side, along) corner index; vertex shader computes
    the screen-space perpendicular and offsets accordingly.  Real
    outlined lines independent of the driver's glLineWidth clamp.
  - screen-space rect shader: HUD + label backgrounds drawn as raw
    GL quads in NDC.  QPainter::fillRect on QOpenGLPaintDevice was
    silently dropping fills across drivers; bypassing it entirely
    via this shader makes backgrounds reliable.  Cull-face is also
    explicitly disabled here — GL_TRIANGLES respects it but the
    line/point primitives don't, so this was the one path needing
    the fix.

setOverlayLines / setOverlayPoints take an inner color, an outline
color, and an extra-pixels-per-side stroke amount.  Lines + points
draw with GL_ALWAYS so measurement annotations stay visible through
geometry; highlight tris stay depth-aware (GL_LEQUAL) so area
shading still tints the surface in place.

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

src/ifcviewer-full/MainWindow.h

@@ -200,7 +200,8 @@ private:
     QString pending_camera_;
     int     pending_benchmark_ = 0;
 
-    AreaMeasurement area_measurement_;
+    AreaMeasurement   area_measurement_;
+    LengthMeasurement length_measurement_;
 };
 
 #endif // MAINWINDOW_H