feat: grade a sparse window's polyline by slope, so phosphor does not switch off on zoom

Intensity grading was gated on the column reduction, which is gated on a
density whose only job is killing the moire beat. Dwell has nothing to do
with that, so narrowing a window past the threshold turned the whole
phosphor model off and a discontinuity went from a third of a flat top's
ink to all of it in one pixel of zoom.

stage_trace_slopes() grades each segment by its own slope instead, with a
measure that agrees with the dense path at the boundary, and reports
itself as FrameStats::slope_traces -- disjoint from column_traces, so a
host asserting graded_traces == column_traces reads the same as before.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

include/plot/render/style.hpp

@@ -202,6 +202,14 @@ struct Style {
   /// bright mark. Counting a row at +1 per segment that touches it instead -- the
   /// obvious thing -- grades a sine's fast middle as brightly as its rails, which
   /// is the pattern inverted.
+  ///
+  /// Below `dense_threshold_spp`, where there are no columns to reduce, this grades
+  /// the **polyline** by each segment's own slope instead (`FrameStats::slope_traces`
+  /// against `graded_traces`) -- the same measure for a sparse window, where a
+  /// segment *is* the sub-column. One switch covers both because the alternative was
+  /// a defect: gated on the reduction, the model switched off as a window narrowed
+  /// past the threshold, and an edge drawn at a third of a flat top's ink jumped to
+  /// all of it in one pixel of zoom.
   bool intensity_grading = false;
 
   /// Sub-buckets per pixel column the gather is asked for when grading: the level