fix(render): one fence per frame for the vertex ring; two qt6 adapter fixes

The persistent-mapped instance ring had three slots and recorded a fence per
upload, so a frame recycled a slot whose fence was recorded a few draws earlier
*in that same frame*: from the fourth of the demos' nine uploads on, each one
waited on the frame's own work. It is free on a driver that retires draws
promptly (the GLFW host, 1.07 ms/frame) and ~4 ms of the Qt widget's 7.11 ms
under llvmpipe, and it grows with the trace and cursor counts this library
exists for.

* The ring grows to cover a frame's upload count, the frame's write cursor is
  the ring index (so a frame writes 0, 1, 2, ... and never a slot twice), one
  fence is recorded after the frame's last draw, and the only wait is at the
  start of the next frame. A wait that times out, or a slot written with no
  fence ever recorded behind it, drops the store to the orphan path -- the
  treatment a refused fence already had -- so nothing is ever written into
  storage the GPU may still be reading.
* New test-only counters (PLOT_RING_STATS, the PLOT_PYRAMID_COUNT_* pattern:
  free functions, no member, so no consumer's layout changes) pin a frame's
  upload and wait counts. The new tests assert the frame's slot offsets are
  distinct, that the ring grew past its floor to cover the frame, and that a
  frame waits at most once per store -- read from the counters and from
  glClientWaitSync itself.
* qt6: print the unresolved entry-point names before the load-failure branch,
  which used to return first and leave the one failure this widget can have
  reporting nothing but a generic message.
* qt6: the cached pointer anchor is now in logical pixels and a move's delta is
  the logical difference scaled by the DPR in effect, so a scale change between
  two move events (the window dragged onto another monitor mid-drag) is no
  longer a cross-scale jump applied as a pan. The anchor moved into a
  toolkit-free unit (adapters/qt6/qt_pointer.hpp) with its own test.

GLFW demo headless: 1.07 -> 0.73 ms/frame. Qt widget under xvfb/llvmpipe:
7.1 -> 3.8 ms at dpr 1.0, 6.7 -> 3.6 ms at 1.25, with both upload paths still
rendering and the demo's scripted pan/box-zoom checks passing at both scales.

adapters/qt6/qt_plot_widget.cpp

@@ -1,4 +1,5 @@
 #include <qt6/qt_plot_widget.hpp>
+#include <qt6/qt_pointer.hpp>
 
 #include <plot/gl/loader.hpp>
 
@@ -114,21 +115,26 @@ void PlotWidget::initializeGL() {
   renderer_.reset();
 
   g_unresolved_count = 0;
-  if (!plot::gl::load(&resolve_via_qt)) {
+  const bool loaded = plot::gl::load(&resolve_via_qt);
+  // Printed before the failure branch, not after it: the names collected here are
+  // precisely what explains a failure -- which 3.3 entry point this context could
+  // not provide -- so a load that failed must report them too, or the one failure
+  // this widget can have says nothing but "missing a required entry point".
+  if (g_unresolved_count > 0) {
+    std::fprintf(stderr,
+                 "[plot::qt6] QOpenGLContext::getProcAddress() did not resolve %u required"
+                 " entry point(s); plot::gl::load() %s for:",
+                 g_unresolved_count, loaded ? "used its dlsym fallback" : "failed");
+    for (u32 i = 0; i < g_unresolved_count; ++i) std::fprintf(stderr, " %s", g_unresolved[i]);
+    std::fprintf(stderr, "\n");
+  }
+  if (!loaded) {
     init_status_ = Status{ErrorCode::Unsupported,
                           "plot::gl::load() failed: the current context is missing a required"
                           " OpenGL 3.3 entry point"};
     std::fprintf(stderr, "[plot::qt6] %s\n", init_status_.message);
     return;
   }
-  if (g_unresolved_count > 0) {
-    std::fprintf(stderr,
-                 "[plot::qt6] QOpenGLContext::getProcAddress() did not resolve %u required"
-                 " entry point(s); plot::gl::load() used its dlsym fallback for:",
-                 g_unresolved_count);
-    for (u32 i = 0; i < g_unresolved_count; ++i) std::fprintf(stderr, " %s", g_unresolved[i]);
-    std::fprintf(stderr, "\n");
-  }
 
   renderer_.emplace();
   Status status;
@@ -206,19 +212,24 @@ Vec2 PlotWidget::to_device(const QPointF& p) const noexcept {
 void PlotWidget::refresh_viewport() { controller_.set_viewport(device_viewport()); }
 
 void PlotWidget::forward_pointer(QMouseEvent* event, bool pressed, bool released) {
-  const Vec2 p = to_device(event->position());
+  // The anchor stays logical (see PointerAnchor): a move that arrives after the
+  // window moved to a display with another scale factor must not measure the gap
+  // between a position taken at the old DPR and one taken at the new one. The
+  // delta is the logical difference scaled by the ratio in effect now, so the same
+  // gesture maps to the same device distance at either scale.
+  const QPointF position = event->position();
+  const f64 dpr = static_cast<f64>(devicePixelRatioF());
+  const Vec2 logical{static_cast<f32>(position.x()), static_cast<f32>(position.y())};
+  const Vec2 p = to_device(position);
+  const Vec2 delta = anchor_.device_delta(logical, dpr);
   input::PointerEvent e;
   e.x = p.x;
   e.y = p.y;
-  if (have_pointer_) {
-    e.dx = p.x - last_x_;
-    e.dy = p.y - last_y_;
-  }
+  e.dx = delta.x;
+  e.dy = delta.y;
   // A press is also a pointer sample: the next move's delta starts here, or the
   // first motion of a drag would include the jump to the click.
-  last_x_ = p.x;
-  last_y_ = p.y;
-  have_pointer_ = true;
+  anchor_.set(logical);
   e.button = button_from(event->button());
   e.mods = mods_from(event->modifiers());
   e.down = any_plot_button(event->buttons());