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.hpp

@@ -18,6 +18,7 @@
 #include <plot/math/vec2.hpp>
 #include <plot/plot/plot.hpp>
 #include <plot/render/renderer.hpp>
+#include <qt6/qt_pointer.hpp>
 
 #include <QOpenGLFunctions>
 #include <QOpenGLWidget>
@@ -80,14 +81,21 @@ namespace plot::qt {
 ///   * the viewport handed to the plot is `round(width() * dpr)` x
 ///     `round(height() * dpr)` (rounded, not truncated, so the last column of the
 ///     FBO is inside the plot), and
-///   * every pointer, wheel and key anchor is multiplied by the same
-///     `devicePixelRatioF()` before it reaches the controller, deltas included.
+///   * every pointer and wheel position is multiplied by the same
+///     `devicePixelRatioF()` before it reaches the controller.
 ///
 /// `device_viewport()` and `to_device()` are the only two places that mapping
 /// exists, so the viewport and the input cannot disagree -- and they must not: a
 /// mismatch renders correctly but pans by the wrong amount, which is invisible on
 /// an unscaled display and wrong on every other one.
 ///
+/// The *anchor* a move's delta is measured from is the one thing kept in logical
+/// pixels: `devicePixelRatioF()` can change between two move events, and the delta
+/// is then a logical difference scaled by the ratio in effect now, never the
+/// difference of two device positions taken at different ratios (see
+/// `PointerAnchor`). A drag that crosses onto a display with another scale factor
+/// therefore pans by the distance the pointer moved, not by a cross-scale jump.
+///
 /// ## Input
 ///
 /// Qt reports levels and edges directly (`QMouseEvent` press/release are edges,
@@ -174,9 +182,10 @@ class PlotWidget : public QOpenGLWidget, protected QOpenGLFunctions {
   /// The widget's rectangle in device pixels: the plot's viewport, and the size
   /// of the framebuffer the frame is drawn into.
   Rect device_viewport() const noexcept;
-  /// A Qt position (logical widget pixels) in device pixels. The one place the
-  /// input mapping exists, so every event is scaled by the same factor the
-  /// viewport is.
+  /// A Qt position (logical widget pixels) in device pixels. The one place a
+  /// pointer position is mapped, so everything the controller sees is scaled by
+  /// the same factor the viewport is -- a move's *delta*, whose ratio can change
+  /// between two events, is scaled by `PointerAnchor` instead.
   Vec2 to_device(const QPointF& p) const noexcept;
   /// Translates one mouse event and hands it to the controller. `pressed` and
   /// `released` are the transitions Qt reports on the event itself.
@@ -191,11 +200,10 @@ class PlotWidget : public QOpenGLWidget, protected QOpenGLFunctions {
   FrameFn frame_;
   Status init_status_;
 
-  /// Last pointer position in device pixels: the deltas the controller pans with
-  /// are the movement since the previous event, which Qt does not report.
-  bool have_pointer_ = false;
-  f32 last_x_ = 0.0f;
-  f32 last_y_ = 0.0f;
+  /// The position a move's delta is measured from, in **logical** pixels: Qt
+  /// reports no delta of its own, and the ratio can change between two events
+  /// (see `PointerAnchor` for why the anchor must not be device pixels).
+  PointerAnchor anchor_;
 
   u64 frames_ = 0;
   f32 last_frame_ms_ = 0.0f;