Add the command line: commands, completion, ghost text and history

`:` opens a vim-style command line. Every command is described once, in
a typed table (name, the short form that still works, aliases, a `!`,
its arguments and what it does), so parsing, completion and the help can
never disagree. Short forms work as in vim (`:e`, `:vs`, `:q!`, `:x`),
and a test checks that no typed name can mean two commands.

Parsing turns a line into a typed Call, checking every argument against
what the app offers (open documents, nodes, colors, styles, settings),
or says what's wrong in words: the usage when an argument is missing,
the choices when a word isn't one, "did you mean :fold?" for a typo.
Documents and nodes match fuzzily (`:b gpl` finds Garden plans).

While typing, a popup above the word lists what can finish it (command
names by prefix, choices, files from the folders on disk, documents and
nodes fuzzily), with Tab and Shift-Tab putting them in the line. Ghost
text after the cursor comes from the history first, like fish's, then
from the best candidate; → or Ctrl-e takes it. Up and Down browse the
lines that start with what was typed. The line itself is a small input
that steps over whole characters, accents and emoji included.

The canvas example opens it with `:` and runs a handful of commands:
:fold, :color, :border, :tag, :focus, :rename, :in (to an Inbox),
:link, :unlink, :find, :version and :q.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

README.md

@@ -58,6 +58,6 @@ cargo fmt     # format the code
 While the app is being built, the examples try out each part:
 
 ```sh
-cargo run --example canvas                         # the canvas with vim keys and which-key
+cargo run --example canvas                         # the canvas, vim keys, which-key, :commands
 cargo test show_canvas -- --ignored --nocapture    # print it at each zoom, as text
 ```