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, 7 insertions, 1 deletions

diff --git a/src/config.zig b/src/config.zig
index 2a764118..c0266e40 100644
--- a/src/config.zig
+++ b/src/config.zig
@@ -127,6 +127,7 @@ pub const leader_path = paths: {
     if (builtins.capabilities.lift) table.set(.Occlusion, null);
     if (builtins.capabilities.lift) table.set(.Parallax, null);
     if (builtins.capabilities.lift) table.set(.Afterglow, null);
+    table.set(.JumpTrail, null);
     if (builtins.capabilities.lift) table.set(.Lift, null);
     table.set(.Motion, null);
     table.set(.InactiveDim, null);
@@ -686,6 +687,9 @@ pub const Runtime = struct {
     /// G14: text that changes glows for a moment in the theme's accent and
     /// fades, a pixel shell's.
     afterglow: bool = false,
+    /// T2: in a terminal, a jump of the cursor leaves a short trail that
+    /// fades in 120 ms (a truecolor terminal's; a pixel shell glides).
+    jump_trail: bool = false,
     /// How long a message stays after the key or click that dismisses it.
     message_linger_ms: u16 = 800,
     /// How long its fall into the row takes, easing out (§8.1: arriving,
@@ -909,7 +913,7 @@ pub const Runtime = struct {
     /// The message timings a setting word sets, each in milliseconds.
     pub const MessageTime = enum { linger, fall, dissolve };
 
-    pub const Toggle = enum { colors, wrap, tree_context_tag_style, tag_bottom, debug, focus_tint, syntax_bold, verbose, message_animation, ligatures, cursor_blink, selection_glow, hover_glow, occlusion, parallax, afterglow };
+    pub const Toggle = enum { colors, wrap, tree_context_tag_style, tag_bottom, debug, focus_tint, syntax_bold, verbose, message_animation, ligatures, cursor_blink, selection_glow, hover_glow, occlusion, parallax, afterglow, jump_trail };
     /// A bundled post pass, a Shadertoy file in shaders/post/: the CRT, and
     /// G4's bloom, vignette and grain (docs/effects.md).
     pub const Scene = enum { crt, bloom, vignette, grain };
@@ -973,6 +977,7 @@ pub const Runtime = struct {
         .{ .word = "Occlusion", .action = .{ .toggle = .occlusion }, .availability = .lift, .doc = "Pane bodies darken faintly toward their edges, on or off." },
         .{ .word = "Parallax", .action = .{ .toggle = .parallax }, .availability = .lift, .doc = "A theme's page pattern moves with the text at a quarter of its speed, on or off." },
         .{ .word = "Afterglow", .action = .{ .toggle = .afterglow }, .availability = .lift, .doc = "Text that changes glows for a moment and fades, on or off." },
+        .{ .word = "JumpTrail", .action = .{ .toggle = .jump_trail }, .doc = "In a terminal, a jump of the cursor leaves a short trail that fades, on or off." },
         .{ .word = "Lift", .action = .lift, .availability = .lift, .doc = "How the focused pane stands off the page: off, shadow, rim or auto." },
         .{ .word = "Motion", .action = .motion, .doc = "How animations move: off, crisp, smooth, bouncy or playful." },
         .{ .word = "InactiveDim", .action = .inactive_dim, .doc = "Fade the unfocused panes' text by a percentage, 0 through 100, 0 off." },
@@ -1711,6 +1716,7 @@ pub const Runtime = struct {
                     .occlusion => "Occlusion on\n",
                     .parallax => "Parallax on\n",
                     .afterglow => "Afterglow on\n",
+                    .jump_trail => "JumpTrail on\n",
                 },
                 .shell => "Shell requested (new panes) fish\n",
                 .tagline_size => "TaglineSize 73\n",