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.

tests/test_qt_pointer.cpp

@@ -0,0 +1,56 @@
+// The Qt adapter's pointer translation, driven directly. The widget's cached
+// pointer anchor is a plain value with no toolkit dependency (see
+// adapters/qt6/qt_pointer.hpp) precisely so the one thing a screenshot cannot show
+// -- which pixel space a drag delta is in -- is testable without a QApplication, a
+// window or a display.
+#include "harness.hpp"
+
+#include <qt6/qt_pointer.hpp>
+
+#include <cmath>
+
+// The defect this pins: the anchor used to be cached in *device* pixels while the
+// next event's delta was a raw difference of device positions, so a DPR change
+// between two move events -- the window dragged onto a monitor with a different
+// scale factor mid-drag -- mixed two pixel spaces. At 1.0 -> 1.25 a logical
+// position of 500 produced a spurious 125 device-pixel delta, which
+// `Controller::pan` applies as a fraction of the range span and which also clears
+// auto scale: a scale change mid-drag would move the plot by 125 pixels' worth of
+// data and silently disarm a live axis.
+PLOT_TEST(pointer_anchor_survives_a_dpr_change_between_move_events) {
+  plot::qt::PointerAnchor anchor;
+
+  // The press caches a logical position while the window is on the 1.0 display.
+  anchor.set({500.0f, 300.0f});
+
+  // The next move arrives after the window moved to the 1.25 display, at the same
+  // logical position: the pointer has not moved, so the delta must be zero -- not
+  // 500 * 1.25 - 500 * 1.0 = 125 device pixels, which is what comparing two device
+  // positions across the scale change reported.
+  const plot::Vec2 still = anchor.device_delta({500.0f, 300.0f}, 1.25);
+  PLOT_CHECK_NEAR(still.x, 0.0, 1e-6);
+  PLOT_CHECK_NEAR(still.y, 0.0, 1e-6);
+
+  // A real movement is the logical delta scaled by the ratio in effect now, in both
+  // directions.
+  const plot::Vec2 moved = anchor.device_delta({503.0f, 295.0f}, 1.25);
+  PLOT_CHECK_NEAR(moved.x, 3.0 * 1.25, 1e-6);
+  PLOT_CHECK_NEAR(moved.y, -5.0 * 1.25, 1e-6);
+
+  // And the anchor follows the sample it just reported: the following move is
+  // measured from there, at whatever ratio that event carries.
+  anchor.set({503.0f, 295.0f});
+  const plot::Vec2 unchanged = anchor.device_delta({503.0f, 295.0f}, 1.0);
+  PLOT_CHECK_NEAR(unchanged.x, 0.0, 1e-6);
+  PLOT_CHECK_NEAR(unchanged.y, 0.0, 1e-6);
+}
+
+// The first sample of a gesture has no anchor to measure from, so its delta is
+// zero however far the position is from the origin: the first motion of a drag
+// must never include the jump to where the drag began.
+PLOT_TEST(pointer_anchor_reports_no_movement_before_its_first_sample) {
+  plot::qt::PointerAnchor anchor;
+  const plot::Vec2 delta = anchor.device_delta({500.0f, 300.0f}, 1.25);
+  PLOT_CHECK_NEAR(delta.x, 0.0, 1e-9);
+  PLOT_CHECK_NEAR(delta.y, 0.0, 1e-9);
+}