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/Measurement.h

@@ -21,6 +21,8 @@
 #define IFCVIEWER_FULL_MEASUREMENT_H
 
 #include <cstddef>
+#include <QString>
+#include <array>
 #include <cstdint>
 #include <unordered_map>
 #include <vector>
@@ -105,4 +107,33 @@ private:
     double                                     total_area_m2_ = 0.0;
 };
 
+// Click-to-place length / angle / area measurement.  Each pick appends a
+// world-space point.  The readout adapts to the point count:
+//
+//   1 point  → "click another point"
+//   2 points → straight-line distance plus axis-aligned ΔX/ΔY/ΔZ
+//   3 points → angle at the middle vertex plus the triangle's area
+//   4+       → polygon area: best-fit-plane shoelace if the points are
+//              near-coplanar (RMS plane distance < 1e-3 of the bounding
+//              box), else fan-triangulated from the first point
+//
+// Clicked points are pushed to the viewport overlay as small dots and
+// the connecting polyline; readouts go to the multi-line HUD.
+class LengthMeasurement {
+public:
+    LengthMeasurement();
+
+    void onPick(ViewportWindow& vp, int x, int y, bool alt);
+    void removeLastPoint(ViewportWindow& vp);
+    void clear(ViewportWindow& vp);
+
+    size_t pointCount() const { return points_.size(); }
+
+private:
+    void rebuildOverlay(ViewportWindow& vp);
+    QString formatReadout() const;
+
+    std::vector<std::array<float, 3>> points_;
+};
+
 #endif // IFCVIEWER_FULL_MEASUREMENT_H