fix(plot,render,gl): live streaming measurements, handed-back GL state

Final whole-branch review fixes.

1 (Important). Plot::measurement() cached on (series pointer, sample count)
only, so a saturated StreamSeries -- whose size() stays pinned while x0()
advances every acquire() -- froze its readout on the first saturated frame
until a cursor moved. The cache key now carries the series' x0() (new
MeasureSlot::last_x0), so a sliding window recomputes and a static series
still does not: asserted with a new macro-gated counter hook
(plot/detail/measure_count.hpp, PLOT_COUNT_MEASURE_RECOMPUTES, wired like the
pyramid and ring counters, absent from the release library). New case
saturated_stream_measurement_tracks_the_sliding_window: saturated ring,
stationary cursors, ramp in absolute x -- zero recomputes for repeated reads
with nothing moving, one per acquire while it slides.

2-3, 9. Spec: 4.3 described a GL_LINE_STRIP path that does not exist (there is
one rasteriser -- instanced anti-aliased quads -- and dense_threshold_spp
selects the gather level, not a primitive); 8 named a PLOT_GL_CHECK() macro
that exists nowhere, and stated no exception policy. The table, the field's
comment and 8 now describe the real gather-level threshold, the real
glGetError() drain plus the stores' drop-to-orphan fallbacks, and the real
policy: library failures return Status/bool, allocating entry points propagate
std::bad_alloc.

4. The frame handed back neither GL_UNPACK_ALIGNMENT (init() sets 1 for the
atlas) nor the clear colour (queried once with a new glGetFloatv entry).
end_frame() restores both and renderer.hpp lists them in the residue
contract.

5. draw() kept a host TextBackend for layout while drawing with its own font.
The renderer now installs its font unconditionally -- strips and labels measure
and draw with one backend -- with a debug-build diagnostic if a different one
was set, and Plot::set_text_backend() is documented as the no-renderer path.
New case renderer_measures_the_labels_it_draws asserts layout, measuring and
the drawn label's ink agree.

6-10. draw()'s style overwrite stated at the call site; the load-sensitive
wall-clock bound in test_measure.cpp replaced by a counter assertion (same
interval, same fold count over a 512x length ratio); dump_frame_ppm drops
noexcept and says why it may allocate; error.txt ignored (not this project's,
not generated here) rather than deleted.

Verified: debug and release build clean (-Wall -Wextra -Wpedantic -Wconversion
-Wshadow), 4/4 ctest entries pass, 130 cases pass (was 127), umbrella and
header self-containment checks pass, and no counter symbol in the release
archive.

src/plot/plot.cpp

@@ -1,4 +1,5 @@
 #include <plot/plot/plot.hpp>
+#include <plot/plot/detail/measure_count.hpp>
 
 #include <algorithm>
 #include <cmath>
@@ -271,11 +272,20 @@ const Measurement& Plot::measurement(u32 id) const {
   const data::Series* series = nullptr;
   if (slot.trace < traces_.size()) series = traces_[slot.trace].series;
   const u64 samples = series != nullptr ? series->size() : 0;
+  // A saturated ring keeps `samples` constant while its window slides: x0() is
+  // what moves, and with it the sample each cursor snaps to and the interval they
+  // measure. Caching on (pointer, count) alone would freeze the readout at the
+  // first saturated frame until a cursor moved.
+  const f64 x0 = series != nullptr ? series->x0() : 0.0;
 
-  // Recompute only when someone moved a cursor, replaced the trace's series, or
-  // grew it. Everything else is a cache hit, which is what keeps a steady frame
-  // from re-evaluating every measure.
-  if (slot.dirty || series != slot.last_series || samples != slot.last_size) {
+  // Recompute only when someone moved a cursor, replaced the trace's series, grew
+  // it, or slid the window under it (x0 above). Everything else is a cache hit,
+  // which is what keeps a steady frame from re-evaluating every measure.
+  if (slot.dirty || series != slot.last_series || samples != slot.last_size ||
+      x0 != slot.last_x0) {
+#ifdef PLOT_COUNT_MEASURE_RECOMPUTES
+    detail::measure_recomputes().fetch_add(1, std::memory_order_relaxed);
+#endif
     slot.result = Measurement{};
     if (series != nullptr && samples > 0 && slot.cursor_a < cursors_.size() &&
         slot.cursor_b < cursors_.size()) {
@@ -287,6 +297,7 @@ const Measurement& Plot::measurement(u32 id) const {
     }
     slot.last_series = series;
     slot.last_size = samples;
+    slot.last_x0 = x0;
     slot.dirty = false;
   }
   return slot.result;