ifcviewer: de-Qt QColor/QPoint/QSet/QElapsedTimer in ViewportWindow

Last round of straight-swap Qt value types in ViewportWindow + its
overlay co-pilot.

  setBackgroundColor(const QColor&)   → (float r, float g, float b, float a)
  QColor   background_color_          → Eigen::Vector4f (linear, 0..1)
  QPoint   {nav_,box_select_,fps_,    } → Eigen::Vector2i
           {section_drag_start_mouse_}
  QSet<int> fps_keys_held_            → std::unordered_set<int>
  QElapsedTimer fps_last_tick_,       → Stopwatch (new header in
               fly_render_clock_,        IfcViewerCore — std::chrono-
               render_thread_local_      backed, exposes the existing
               timers in render()        QElapsedTimer .start/.restart/
                                         .elapsed/.nsecsElapsed surface)

Also propagates the QPoint → Eigen::Vector2i change through
OverlayRenderer::encodeMarquee since the marquee corner coords flow
through that interface.

API-level helpers:
  toV2i(QPoint)        — small inline in ViewportWindow.cpp, isolates
                         the QMouseEvent→Vector2i conversion at the
                         five mouse-event handlers
  Stopwatch.h          — new file, IfcViewerCore. Same call shape as
                         QElapsedTimer; backed by std::chrono::steady_clock.

QSet method swaps:
  .isEmpty() → .empty()
  .contains(k) → .count(k)   (C++17, no std contains() until C++20)
  .remove(k)   → .erase(k)

Eigen::Vector2i doesn't have .manhattanLength(); the box-select drag
threshold uses std::abs(diff.x()) + std::abs(diff.y()) inline.

Bonsai side: View.cpp's setBackgroundColor wrapper now decomposes the
QColor into floats at the call site (kept locally so the bonsai UI
keeps its QColor-driven theming).

Closes #81 + the QElapsedTimer half of #83. QTimer
(pivot_indicator_hide_timer_) still uses Qt — it needs the host's
scheduleOnce mechanism that lands with #85.

Builds: desktop / bonsai / web all green. Tests 100/100.

src/ifcviewer/ViewportWindow.h

@@ -21,13 +21,13 @@
 #define WGPUVIEWPORTWINDOW_H
 
 #include <QWindow>
-#include <QColor>
-#include <QElapsedTimer>
-#include <QPoint>
-#include <QSet>
-#include <string>
 #include <QTimer>
 
+#include <string>
+#include <unordered_set>
+
+#include "Stopwatch.h"
+
 #include <webgpu/webgpu.h>
 
 #include <Eigen/Dense>
@@ -89,7 +89,7 @@ public:
     void  onToolModeChanged(int tool_mode) override;
     void  onToolBackspacePressed() override;
 
-    void setBackgroundColor(const QColor& color);
+    void setBackgroundColor(float r, float g, float b, float a = 1.0f);
 
     // Queue a sidecar path to be loaded after wgpu init completes. Safe to
     // call before the window is exposed. The path is resolved against the
@@ -811,8 +811,8 @@ private:
     // release: plain → replace, Shift → add, Ctrl → remove.
     bool                       box_select_armed_      = false;
     bool                       box_select_active_     = false;
-    QPoint                     box_select_start_pos_;     // logical px
-    QPoint                     box_select_current_pos_;   // logical px
+    Eigen::Vector2i                     box_select_start_pos_;     // logical px
+    Eigen::Vector2i                     box_select_current_pos_;   // logical px
     Qt::KeyboardModifiers      box_select_press_mods_ = Qt::NoModifier;
     static constexpr int       kBoxSelectThresholdPx  = 5;
     // R32UInt staging for the rect-pick. Sized to the largest rect we've
@@ -824,7 +824,7 @@ private:
     // the press fall through to the orbit/pan handlers.
     bool                      section_drag_active_  = false;
     int                       section_drag_index_   = -1;
-    QPoint                    section_drag_start_mouse_;
+    Eigen::Vector2i                    section_drag_start_mouse_;
     Eigen::Vector3f                 section_drag_start_origin_;
     // Mirrors GL ViewportWindow::hitTestSectionGizmo: returns the index of
     // the plane whose arrow gizmo is within grab_px of (x, y), or -1.
@@ -858,7 +858,8 @@ private:
     // thread per model.
     mutable std::atomic<int> hiz_trace_budget_{0};
 
-    QColor background_color_ = QColor("#202329");
+    // 0x20 / 0xff ≈ 0.125, 0x23 / 0xff ≈ 0.137, 0x29 / 0xff ≈ 0.161.
+    Eigen::Vector4f background_color_ = {0.125f, 0.137f, 0.161f, 1.0f};
 
     // Camera (orbit, right-handed Y-up world → wait, BIM is +Z up).
     // Mirrors the GL viewport's defaults; mouse navigation lands later.
@@ -878,9 +879,9 @@ private:
     // exit via Esc (also any unrelated key click) — recenter the cursor
     // back at fps_press_center_ so the orbit camera resumes cleanly.
     bool         fps_mode_                    = false;
-    QSet<int>    fps_keys_held_;
-    QElapsedTimer fps_last_tick_;
-    QPoint       fps_press_center_;
+    std::unordered_set<int> fps_keys_held_;
+    Stopwatch    fps_last_tick_;
+    Eigen::Vector2i       fps_press_center_;
     bool         fps_ignore_next_mouse_move_  = false;
     // Fly base speed in m/s at no-modifier (Shift gives a 5× boost). Default
     // 5.0 matches GL fps_move_speed_. Scrollwheel in fly mode adjusts this
@@ -892,7 +893,7 @@ private:
     // print dt of each fpsIntegrate call and the prior render's elapsed
     // ms. Off by default (env-gated) so the normal log stays clean.
     bool         fly_debug_                   = false;
-    QElapsedTimer fly_render_clock_;
+    Stopwatch    fly_render_clock_;
 
     // Click-and-track diagnostic: when a pick lands, stash the chunk
     // that holds the picked object. driveStreamingLoads watches for that
@@ -1064,8 +1065,8 @@ private:
     // LMB-click-without-drag picks the object under the cursor. No
     // Blender/Maya preset awareness yet — that arrives with AppSettings.
     Qt::MouseButton nav_active_button_ = Qt::NoButton;
-    QPoint          nav_last_pos_;
-    QPoint          nav_press_pos_;
+    Eigen::Vector2i          nav_last_pos_;
+    Eigen::Vector2i          nav_press_pos_;
     bool            nav_dragged_       = false;
 
     // Benchmark mode. setBenchmarkFrames(N) arms it; render() integrates the