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, 17 insertions, 0 deletions

diff --git a/docs/effects.md b/docs/effects.md
index 6a283e45..02602a23 100644
--- a/docs/effects.md
+++ b/docs/effects.md
@@ -247,3 +247,20 @@ Never on a tag or a grip (bodies only), the cursor's cell or a selection;
 a body whose first line moved (a scroll) starts over instead of lighting
 up whole. It runs on the continuous path while anything glows, and costs
 a compare of the grid a frame while on, nothing while off.
+
+## Terminal track (§9.2)
+
+Audit, after decision 2 (keep every whimsical effect and polish it, never
+remove it): the six core-composed character transitions (ascii, edges,
+fall, wave, scramble, typewriter) and curtain stay as they are, each
+bounded to its pane's final rectangle and each cell's last sample its
+canonical one; slide and vertical are the tty compositor's and follow the
+Motion flavour (sxrrwplk); zoom stays (nearest-cell scaling, its look is
+the effect); dissolve swaps each changed cell once at its threshold;
+message fades and the theme's chrome fade are the core's (the latter in
+OKLab since G10), and InactiveDim (T5's unfocused dim) works in a grid as
+it does in a pixel shell.
+
+| # | effect | switch | state | review material | verdict |
+|---|---|---|---|---|---|
+| T2 | cursor jump trail | `JumpTrail on\|off` (off) | a jump of three cells or more leaves 3 to 5 cells of trail behind the cursor, their ground drawn toward the text's colour in OKLab, most beside it, fading in 120 ms; truecolor terminals (vaxis's answer or COLORTERM); never the cursor's cell, a selection or outside the focused body; off under Motion off; a few cells a frame | .scratch/render/tty/trail/ (pyte replay of a real tty session at 60 fps: trail-forge.mp4, zooms) | pending |