ifcviewer-full: console-print accumulating coplanar-patch area tool

Adds a click-to-measure area mode triggered by Ctrl+Shift+A.  Each LMB
click expands the picked triangle into its connected coplanar patch
(BFS over shared edges, dot(normal, seed) > 0.9999); re-clicking
removes that patch; Alt+LMB skips expansion for a single triangle.
Picks across different meshes accumulate as separate patches.

ViewportWindow gains pickMeshLocalAt (screen pick → mesh-local hit
via inverse composed transform) and a tool-mode pattern mirroring
the section tool (toggleAreaTool, surfacePickedInTool signal,
areaToolToggled signal, Esc to exit).  Per-mesh adjacency is built
lazily on first pick of each mesh and dropped on tool toggle.

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 <cstdint>
+#include <unordered_map>
+#include <unordered_set>
 #include <vector>
 
 class ViewportWindow;
@@ -35,4 +37,56 @@ class ViewportWindow;
 double volumeOfObjects(ViewportWindow& vp,
                        const std::vector<uint32_t>& object_ids);
 
+// Click-to-accumulate area measurement.  Each pick resolves the screen
+// click to a (model, mesh, triangle) using ViewportWindow's primitives,
+// expands it into the connected coplanar patch (BFS over shared edges,
+// dot(normal, seed_normal) > 0.9999), then either adds or removes that
+// patch from the running set depending on whether the seed triangle was
+// already in.  Alt-click skips the BFS expansion (single-triangle).
+// Picks across different meshes are kept as separate patches and their
+// areas are summed.
+//
+// State is cleared on construction, on clear(), and is expected to be
+// reset by the host (e.g. when the viewport's area tool toggles off).
+class AreaMeasurement {
+public:
+    AreaMeasurement();
+
+    // Main entry point: handle one click in area-tool mode.  alt = true
+    // suppresses BFS expansion.  Logs the per-click delta and running total
+    // via qInfo.  Misses are silent.
+    void onPick(ViewportWindow& vp, int x, int y, bool alt);
+
+    // Wipe all accumulated triangles and per-mesh adjacency caches.
+    void clear();
+
+    double totalArea() const { return total_area_m2_; }
+    size_t triangleCount() const { return selected_.size(); }
+
+private:
+    // Cached per-mesh data: triangles + edge→triangles adjacency.  Keyed
+    // by (model_id << 32) | mesh_id.  Filled lazily on first pick of that
+    // mesh, dropped on clear().
+    struct MeshCache {
+        std::vector<float>    positions;     // 3 * N_verts
+        std::vector<uint32_t> indices;       // 3 * N_tris
+        std::vector<float>    tri_normals;   // 3 * N_tris (unit, mesh-local)
+        std::vector<float>    tri_areas;     // N_tris
+        // edge_key (min<<32 | max) → list of triangle indices touching it.
+        std::unordered_map<uint64_t, std::vector<uint32_t>> edges;
+    };
+    MeshCache* meshCache(ViewportWindow& vp, uint32_t model_id, uint32_t mesh_id);
+
+    // Selection key: (uint64) packing model_id (high 24), mesh_id (mid 24),
+    // triangle index (low 16).  16 bits is enough — meshes with > 65k tris
+    // are rare and the streamer chunks them anyway.
+    static uint64_t triKey(uint32_t model_id, uint32_t mesh_id, uint32_t tri) {
+        return (uint64_t(model_id) << 40) | (uint64_t(mesh_id) << 16) | uint64_t(tri);
+    }
+
+    std::unordered_map<uint64_t, MeshCache> mesh_cache_;
+    std::unordered_set<uint64_t>            selected_;
+    double                                  total_area_m2_ = 0.0;
+};
+
 #endif // IFCVIEWER_FULL_MEASUREMENT_H