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>

docs/building-and-testing.md

@@ -41,11 +41,18 @@ ctest --test-dir build/debug --output-on-failure
 
 ctest registers four tests: `plot_tests` (the test binary), the two header/umbrella build
 checks (`plot_headers_self_contained`, `plot_umbrella_covers_public_headers`) and
-`plot_umbrella_check`. The `plot_tests` binary itself runs **350 cases** in the `linux-debug`
-preset; its harness prints one line per case and a final `350 cases, 0 failed` summary line.
+`plot_umbrella_check`. The `plot_tests` binary itself runs **377 cases** in the `linux-debug`
+preset; its harness prints one line per case and a final `377 cases, 0 failed` summary line.
 The case counts per file are the `PLOT_TEST(...)` declarations under `tests/` — recount them
 (`grep -c '^PLOT_TEST(' tests/test_*.cpp`) after adding or removing cases.
 
+`PLOT_TEST_FILTER=<substring>` runs only the cases whose name contains that substring, and
+is for working *on* a case rather than for checking the library: most of the render cases
+create their own EGL context and read pixels back, so the whole binary is far slower than
+the one case being edited and its one line of measured output scrolls past 376 others. The
+summary line still counts the whole suite, so a filtered run says `377 cases, 0 failed`
+having run one — it is never evidence that the suite passes. Unset, every case runs.
+
 ## Install layout
 
 `cmake --install` follows `GNUInstallDirs`, so every path honours the standard cache
@@ -119,7 +126,7 @@ per-file table and the floor verdict live in [docs/coverage.md](coverage.md), wh
 `tools/coverage.sh` rewrites from the very run that measured them. Re-run the command for the
 live numbers.
 
-**One thing to know about the coverage preset:** it builds **333 of the 350 cases**. The 17
+**One thing to know about the coverage preset:** it builds **360 of the 377 cases**. The 17
 that are missing are the adapter cases that need an adapter target to exist — 11 in
 `test_imgui_widget.cpp` and 6 in `test_glfw_host.cpp` — and the coverage preset turns the
 adapters off. The toolkit-free adapter seams still run there: `test_qt_pointer.cpp` (5 cases)