T2: JumpTrail, a terminal cursor's jump leaves a short fading trail

JumpTrail on|off (off; the tty shell's, §9.2 T2): a jump of three cells or
more leaves three to five cells of trail behind the cursor along its path,
their ground mixed toward the text's colour in OKLab (most beside the
cursor), fading out over 120 ms. Truecolor terminals only (vaxis's answer,
or COLORTERM truecolor/24bit); never the cursor's own cell, a selection or
anything outside the focused pane's body; none under Motion off. It rides
the continuous path (Pardes.shell_continuous) while it lasts; a few cells
of output a frame. docs/effects.md opens the terminal track with the audit
under decision 2 (keep and polish, remove nothing).

Tests: a ten-cell jump lights 3 to 5 cells behind the cursor on its row,
gone by 120 ms; a one-cell step and a terminal without truecolor light
none. Feel review: .scratch/render/tty/trail/ (a real tty session replayed
through pyte at 60 fps).

1 files changed, 3 insertions, 0 deletions

diff --git a/docs/config.md b/docs/config.md
index c66b124e..1d83bd8e 100644
--- a/docs/config.md
+++ b/docs/config.md
@@ -582,6 +582,9 @@ Parallax on      a theme's page pattern (lapis's dots) moves with the text
                  at a quarter of its speed
 Afterglow on     text that changes glows for a moment in the theme's accent
                  and fades (250 ms), never on a tag, the cursor or a selection
+JumpTrail on     in a terminal, a jump of the cursor of three cells or more
+                 leaves a trail of a few cells that fades in 120 ms
+                 (truecolor terminals; a pixel shell glides instead)
 CursorBlink on   the cursor blinks, solid while typing and half a second
                  after, eased at each edge, solid after 10 idle seconds
 GripWidth 150    the grip's button and the scrollbar under it, percent of the