sl0ptty: the border is the button — click an edge to split toward it

The per-pane '+' is gone. It was one glyph for a verb with four directions, so
it always split into columns; it taxed every frame three columns of title
forever; and there were N of them on screen for one verb.

Clicking a pane's border now splits toward that border: the side you click is
the side the new pane appears on. Hovering arms the edge (it goes heavy) and
draws a dashed line exactly where the new boundary would land, so the gesture
explains itself and an idle frame stays quiet.

  ╭──────────────────── shell ─────────────────────╮
  ┃                        ╎                       │
  ┃                        ╎                       │     hovering the left
  ╰────────────────────────────────────────────────╯     border

  │╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌╌│     hovering the bottom
  ╰━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━╯     border

The top border keeps its other job: a click splits upward, a drag moves the
pane. A press that never moves is a click, which is a distinction the drag
machine already had to make.

The guide has to be painted after the pane's content, not with the frame: the
dashed line crosses the terminal's own cells, and the first version was drawn
underneath them and was invisible.

Two old tests changed rather than the code, both because the gesture set is
better now: 'beside the split button' is a drag handle, and 'dropping a pane
on itself does nothing' is a click, which splits. The replacement asserts what
actually needs asserting — a drag that wanders and returns changes nothing.

387 checks green (360 headless + 27 live).

1 files changed, 4 insertions, 2 deletions

diff --git a/README.md b/README.md
index b51b7a0..3bda04b 100644
--- a/README.md
+++ b/README.md
@@ -52,8 +52,10 @@ sl0ptty cmd '{"cmd":"panes"}'
 | `C-a x` / `C-a d` / `C-a q` | close pane / detach / quit |
 | `C-a C-a` | send a literal `C-a` |
 
-The mouse works throughout: click a pane to focus it, a tab to switch, `+` on a
-frame to split, `+tab` to open one, a collapsed header to expand it, and a
+The mouse works throughout. **Click a pane's border to split toward it** — the
+side you click is the side the new pane appears on, and hovering a border arms
+it and shows a dashed line where the split would land. Click a pane to focus
+it, a tab to switch, `+tab` to open one, a collapsed header to expand it, and a
 pane's own buttons to talk to the program inside it. **Drag a pane's title bar
 onto another pane to swap them, or drag the gap between two panes to move the
 boundary.** Focus follows the pointer (`focus_follows_mouse`, on by default,