Esc alternates between the last two panes; Toggleterm is gone

Esc ran Toggleterm, which hopped between the newest DOC and the newest
TERMINAL. That distinction never earned its keep. It made Esc unpredictable —
which of three panes you landed on depended on their kinds, not on where you
had been — and it could not alternate between two files at all, which is the
case you hit most. Editing two files, Esc did nothing.

The replacement already existed. Last (SPC j j) is "the pane you were in
before this one, whichever it was": it walks the jump stack for the newest
entry naming a different pane and restores its line and column. So Esc, and
Shift-Esc in tty, now run Last, and Toggleterm is deleted rather than renamed
— a third implementation of "go to the other pane" was the thing to avoid.
SPC w t goes with it; the w group is the four directional moves, and the jump
group already had SPC j j.

Held down, Esc alternates. Two files, a file and its shell, a file and a
+Search — all the same, because Last has no notion of kind to get wrong.

This depends on the swap in the same series: Last reads the stack backwards,
and until hopping stopped appending, the pane you came from could fall off it.

windownav.snap needed only its keys and prose changed — its golden did not
move at all, which is the useful evidence here: for the one scenario the old
builtin handled well, Last produces an identical focus sequence.

Coverage for what it did not handle is new: a unit test opens a second FILE by
looking its name and asserts Esc alternates between two panes of the SAME
kind, which is the case that used to be a no-op.

Docs follow: tutor.txt, docs/helix-keys.md, docs/design.typ, and the builtin
index goldens, which are now one row shorter.

75/75 snapshots, both unit suites, and the macOS ABI build all pass.

1 files changed, 3 insertions, 2 deletions

diff --git a/docs/design.typ b/docs/design.typ
index c6f0e73e..75c89812 100644
--- a/docs/design.typ
+++ b/docs/design.typ
@@ -201,8 +201,9 @@ the source (cursor row kept visible); dying pane's weight absorbed by one
 sibling; emptied column hands width to a neighbor; border-drag resize on a
 pane's own trailing edge (v and h), hover shows `╎`/`╌` glyph-only hints;
 move-drag via the gutter box with preview; Alt-n new shell below, Alt-c move
-pane to new column; Ctrl-w h/j/k/l directional focus; body-normal Esc toggles
-between the latest document and terminal (the same builtin as SPC w t);
+pane to new column; Ctrl-w h/j/k/l directional focus; body-normal Esc hops to
+the pane you were in before this one, alternating between two (the same
+builtin as SPC j j);
 `--tty` single-pane mode.
 
 *Mouse.* Left: select (block, stays highlighted after drag, pins cursor,