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, 4 insertions, 4 deletions

diff --git a/test/snapshots/argv.golden b/test/snapshots/argv.golden
index 02b5e2e6..1109132c 100644
--- a/test/snapshots/argv.golden
+++ b/test/snapshots/argv.golden
@@ -1,6 +1,6 @@
 == snap noargs grid=100x30 cursor=4,4
 |New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
-|$ /tmp/pardes-snap/argv/cwd New Del
+|$ /tmp/pardes-snap/argv/cwd                                              New Del
 |  $ echo ARGV''TTY
 |  ARGVTTY
 |  $
@@ -31,7 +31,7 @@
 |
 == snap filearg grid=100x30 cursor=7,2
 |New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
-|  /tmp/pardes-snap/argv/cwd/notes.txt Save New Del
+|  /tmp/pardes-snap/argv/cwd/notes.txt                               Save New Del
 |     3 line 3
 |     4 line 4
 |     5 line 5
@@ -62,7 +62,7 @@
 |
 == snap filearg-n3 grid=100x30 cursor=7,2
 |New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
-|  /tmp/pardes-snap/argv/cwd/notes.txt Save New Del
+|  /tmp/pardes-snap/argv/cwd/notes.txt                               Save New Del
 |     1 line 1
 |     2 line 2
 |     3 line 3
@@ -93,7 +93,7 @@
 |
 == snap dirarg grid=100x30 cursor=4,4
 |New Newcol Find Grep Help Tutor Dump NextColor Debug Kill
-|  /tmp/pardes-snap/argv/cwd/sub New Del
+|  /tmp/pardes-snap/argv/cwd/sub                                          New Del
 |
 |  inside.txt
 |