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, 17 insertions, 20 deletions

diff --git a/test/snapshots/windownav.snap b/test/snapshots/windownav.snap
index 5652116b..c9de55ac 100644
--- a/test/snapshots/windownav.snap
+++ b/test/snapshots/windownav.snap
@@ -1,12 +1,11 @@
 # RULE: moving focus is a BUILTIN like any other, so there is ONE
 # implementation and every route reaches it. SPC w h/j/k/l run Left/Down/Up/
-# Right — the geometric focusDir the Ctrl-w prefix also runs — and SPC w t
-# plus body-normal Esc run Toggleterm, the hop between the most recently
-# focused DOCUMENT (a file, an image, a +Search/+Help output buffer: things
-# you READ) and the most
-# recently focused TERMINAL. "Most recent" is the focus history the core
-# already keeps to hand focus back when a pane closes, so the hop back is the
-# same key. Focus shows up in the golden's `cursor=` header: that is the
+# Right — the geometric focusDir the Ctrl-w prefix also runs — and SPC j j
+# plus body-normal Esc run Last, the hop to the pane you were in BEFORE this
+# one, whichever kind it was. It reads the focus history the core already keeps
+# to hand focus back when a pane closes, and going somewhere is itself recorded,
+# which is what makes the hop back the same key. Focus shows up in the golden's
+# `cursor=` header: that is the
 # ACTIVE pane's cursor, so each snap is named by the pane it should land in.
 file w.txt alpha\nbeta\ngamma
 start 30 100 w.txt
@@ -15,22 +14,20 @@ wait 8000 w.txt
 stable 700 20000
 # A FILE boots alone now, so the terminal every hop below needs is opened here:
 # SPC c n (Newcol) puts it in a column of its own on the right and focuses it,
-# SPC w h comes straight back. WHAT THIS COST: the first SPC w t used to run
-# against a shell that had never been focused, which is Toggleterm's fallback
-# scan; nothing reaches that branch any more, because every way a script has of
-# making a terminal also focuses it.
+# SPC w h comes straight back — which also puts the shell on the stack, and Last
+# only ever names a pane that is on it.
 key space c n
 stable 700 15000
 key space w h
 stable 400 5000
 # left column = the file, right column = that shell; the file has focus
 snap start
-# SPC w t: the hop to the most recently focused TERMINAL, the one just opened
-key space w t
+# SPC j j: the pane before this one, which is the shell just opened
+key space j j
 stable 400 5000
 snap to-term
-# ...and back, because the file is now the latest document
-key space w t
+# ...and back, because going there made the file the pane before THAT
+key space j j
 stable 400 5000
 snap back-to-file
 # A bare Esc from a body in normal mode dispatches that SAME builtin: file to
@@ -63,12 +60,12 @@ snap left
 key space w j
 stable 400 5000
 snap down
-# SPC w t from a terminal: the file, which is the only document here
-key space w t
+# SPC j j from a terminal: the file, which is where SPC w h left off
+key space j j
 stable 400 5000
 snap toggle-doc
-# ...and back to the LATEST terminal — the one just left, not the launch shell
-# in the right column, which was focused earlier
-key space w t
+# ...and back to the shell just left, not the launch shell in the right column,
+# which was focused earlier
+key space j j
 stable 400 5000
 snap toggle-back