feat: a dense trace's columns are graded by dwell, so density reads as brightness

Stage 1 cured the moire by collapsing a dense gathered window onto one vertical
segment per pixel column; this is the information that collapse throws away. A
column's min and max are the same whether its samples are spread evenly or piled
at its two ends, so the flat parts of a square wave and the edges between them
drew as one ink. Style::intensity_grading, which the library leaves off and this
app turns on at the first tick (the renderer does not exist until the area is
realized), grades each column by the time the signal spent at each row -- the
digital-phosphor look.

frame path: gains a graded= field, which must equal columns=: grading is a way of
drawing the reduction, not a third path. UDPSCOPE_NO_PHOSPHOR=1 puts the style
back to the library's default and is the A/B, since a frame mean cannot attribute
its own cost -- measured on the fast mock, draw 3.24 ms graded against 2.30 ms,
with segments 1340/4157 against 131/4306 over the same lod=453 frames. That A/B
is also what found the level-floor regression now fixed upstream.

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

README.md

@@ -971,6 +971,39 @@ staircase, 0 short, worst drawn-vs-quoted 0.0000 px` either way. `frame path:`'s
 `columns=` field is the reading that separates "the segment count collapsed because the
 reduction ran" from "the window got narrower".
 
+**Those columns are then graded by dwell, which is the rest of the answer.** A reduced column
+is honest about *where* the signal went and silent about *how long it was there*: its min and
+max are the same whether its samples fill the span evenly or pile at its two ends, so a ramp
+and a pulse are one flat band. `Style::intensity_grading` — off in the library, which udpscope
+turns on at the first tick, since the renderer does not exist until the area is realized —
+draws each column graded instead: bright where the signal sits, faint where it only passes
+through, the way a digital-phosphor scope reads. The distribution cannot be recovered from the
+band, so the gather is asked at a level finer than the pixel grid and the dwell read from those
+sub-buckets; the measure is dwell *per row*, each segment depositing one unit spread over the
+rows it spans, and normalisation is against the column's own peak. See the upstream
+`docs/data-and-performance.md` for the five things that carry it and for the two that were
+wrong first (counting rows at +1 each inverts the pattern, grading a sine's fast middle as
+brightly as its rails; a frame-wide scale needs a 33 MB density image *and* fades the trace out
+exactly where it moves fastest).
+
+It costs about a millisecond a frame and the A/B is the only way to read that, because a frame
+mean cannot attribute its own cost: **`UDPSCOPE_NO_PHOSPHOR=1` puts the style back to the
+library's default**, and the same probe run on the fast mock (`--rate 1300000 --batch 2000`)
+reads `draw 3.24/21.43 ms` with the grading against `draw 2.30/25.76 ms` without, over the same
+`lod=453` frames. **`frame path:`'s `graded=` must equal `columns=`** — grading is a way of
+drawing the reduction and not a third path, so the two counts agreeing is what says the look
+reaches every reduced frame (`columns=1166 graded=1166` in that run, `graded=0` under the env
+var). The segment count rises with it, from a mean of 131 to 1340, which is the gradient's
+tiers: alpha is quantised into steps because the line pipeline's colour is a uniform and a
+per-segment alpha is inexpressible, and each step is its own draw call.
+
+The one performance trap here is worth stating because only the A/B found it. Asking for a
+finer level drops straight to **raw samples** once a sub-column falls under one pyramid bucket
+(64 samples), which at the run's 172 samples a pixel it did: `raw=1154 lod=0` and 6.4 ms a
+frame, i.e. grading undoing the very reduction it draws. The finer ask is floored at level 1
+whenever the flat path would have used the pyramid, so what degrades is the *look* — fewer
+sub-columns than asked for — and never the path.
+
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