sl0p: fix double-click-to-rename, which never fired

Two independent bugs, either of which alone made the feature dead code.

1. WRONG EDGE. The check was `details.edge == Some(PaneEdge::Top)`, but
   get_edge_at_position() quadrant-splits a pane and only ever returns the four
   CORNER variants - PaneEdge::Top exists in the enum and is never produced. A
   title-row click reports as TopLeft/TopRight, so the condition was never true.
   Replaced with Pane::position_is_on_title_row(), which tests the geometry
   directly the same way position_is_on_frame() does for the top border.

2. RESET ON RELEASE. Click recording reset the tracker on any event that was
   not a frame press - which includes the RELEASE between the two clicks of a
   double-click. The sequence is press/release/press, so the count went
   1 -> reset -> 1 and never reached 2.

The tests passed anyway because they hand-built a MouseEventContext with
edge: Some(PaneEdge::Top) and called the decision function directly. They
asserted my assumption rather than the behaviour, and exercised neither the
edge lookup nor the click sequence.

Click recording is now a small pure helper (record_frame_click) covered by
tests that replay real event sequences: press/release/press is a double-click,
motion in between does not break it, a press off the frame does break it, and
two clicks at different positions are not a double-click. The decision tests
now use the corner edge value that is actually produced.

Baselines held: zellij-server 1428/30 (7 new tests).

0 files changed, 0 insertions, 0 deletions