fix: a dense trace is drawn one pixel column at a time, so it cannot moiré

At a high rate, a high frequency or on noise the plots banded visibly and the bands
crawled under a pan. Three things compound to produce it, none a bug alone: a
pyramid bucket spans one to two pixel columns, Envelope emits two points per bucket
so each contributes a near-vertical hairline and a near-vertical connector, and
every one lands at a fractional x because a trace is deliberately not snapped --
so a column's brightness beats against the pixel grid, and a pan sweeps the beat.

The cure is upstream (deps/plotter, pin moved): draw_traces reduces such a window
onto the pixel grid, one snapped vertical segment per column from that column's
lowest to its highest value. One sub-pixel phase for the whole trace, so there is
nothing left to beat, and the geometry is bounded by axes.w rather than by the
buckets.

frame path: gains a columns= field, which is what separates "the segment count
collapsed because the reduction ran" from "the window got narrower". On the fast
mock at 3 kHz over 20 s: columns=1200, segments 2041/2237 over 1201 painted frames,
against columns=0, segments 10960/216093 over 263 before -- the same run at 4.6x
the frame rate, with trace ink: clean either way.

Also corrects the documented mock invocation: --batch 20000 exceeds the datagram
limit and the mock dies with Errno 90.

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

README.md

@@ -939,6 +939,38 @@ into the tick, the plot's own draw and the whole GTK paint, and report the segme
 and whether the LOD path was taken at all. A segment count in the millions is this defect
 returning.
 
+**A dense trace is drawn one pixel column at a time, which is what cured the moiré.** At a
+high sample rate, a high signal frequency or on a noisy signal the plots showed strong,
+*crawling* banding — and three things compounded to produce it, none of which is a bug on its
+own. `MinMaxPyramid::level_for()` sizes a bucket at one to two pixel columns; `Envelope`
+emits two points per bucket, so each bucket contributes a near-vertical min→max hairline
+*and* a near-vertical connector to the next, about 3.6 near-vertical hairlines per column per
+trace; and every one of them lands at a *fractional* x, because a trace is deliberately not
+snapped. With src-alpha compositing a column's brightness is then a non-linear function of
+the sub-pixel phases overlapping it, the phase pattern repeats at the beat between the bucket
+pitch and the pixel pitch, and a pan sweeps the beat. It needs a *thin ribbon* to be visible:
+a window whose buckets each span several periods paints a saturated block and hides it
+entirely, which is why it showed up on fast and noisy signals and not on a slow sine at a
+whole-window view.
+
+`deps/plotter`'s `Renderer::draw_traces` now reduces such a window onto the pixel grid: one
+exactly vertical segment per integer column, from the lowest to the highest screen y any
+gathered point put in it, flushed in framebuffer pixels with the snap on. The whole trace
+then has **one** sub-pixel phase, so there is nothing left to beat. See the upstream
+`docs/data-and-performance.md` for the four rules it keeps (a hole stays a hole, disjoint
+neighbours are joined so a fast edge keeps its join, a span under the stroke is drawn *as*
+the stroke, and the gate is the gathered density rather than the LOD level) and for why this
+is the one carve-out in `u_snap`'s rule that chrome may be snapped and data may not.
+
+It is also a large speed-up, because the geometry becomes O(axes.w) instead of O(buckets).
+Measured on the fast mock (`--rate 1300000 --batch 2000 --freq 3000`, a 20 s run): `frame
+path: raw=1 lod=262 columns=0 frames, segments 10960/216093` and **263 painted frames**
+before, `raw=2 lod=1198 columns=1200 frames, segments 2041/2237` and **1201 painted frames**
+after — the same run at 4.6× the frame rate, with `trace ink:` reading `0 collapsed, 0
+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".
+
 **The panels and the toolbar.** The header bar carries only what you reach for
 *while watching a signal*: the time window, pause/resume, the trigger toggle, the
 side-panel toggle and an overflow menu. Those