Answer "what can I press" in one keypress

The Shortcuts settings page answers "how do I change this", which is
worth opening a window for. This answers the other question, the one
you have with your hands already on the keyboard, so it is an overlay
on SUPER + / and the same key closes it.

It reads Keybinds.grouped() rather than a written-down list, so a
shortcut rebound in Settings shows its new chord here with nothing kept
in sync. A cheatsheet that lies is worse than none: it gets consulted
exactly when somebody does not already know.

Three columns, balanced by how many shortcuts each category holds. The
first attempt used a Flow, which wraps into as many columns as it likes
and made 120 binds across six uneven categories unreadable; it also
sized the card from a child that filled it, which is a circular binding
and produced a card taller than the display with its contents running
off the bottom. Both were found by looking at it rather than by a test,
which is the argument for looking at it.

Fixes a real bug on the way past: luaChord and formatChord appended the
key unconditionally, so the window switcher's modifier-only release
bind became "SUPER + " with a dangling separator. That matched neither
the chord keybinds.lua binds nor the one an override is keyed by, so
that bind could never be rebound and had no category -- it was sitting
in a seventh group of its own, which is how it was noticed.

README.md

@@ -113,7 +113,7 @@ docs/           Settings reference, and the design specs behind the work
 
 ## Tests
 
-140 of them, under `tests/`. Run the lot, or a subset by pattern:
+141 of them, under `tests/`. Run the lot, or a subset by pattern:
 
 ```sh
 panama test           # everything