nodes: names that cannot be confused, and a search of nothing

Three small things the docs pass turned up.

A search of nothing but punctuation — `!!!`, `--` — gave the FTS query no
terms, and the list handler then fell through to browsing with no parent,
which answered with the spaces as if they were matches. It now finds nothing.

Two spaces could have the same name: the slug was made unique (`work-2`) but
the name kept, so the bar showed two pills that read alike and `:space work`
picked the first. A new space, a space renamed, and a directory made at the
root of the mount are refused when another space has that name in any case or
that slug. Folders and documents may still share names; they are told apart by
where they sit.

A name had no ceiling. A file is called by its name on the mount, where most
filesystems stop at 255 bytes, so a document, folder, file or space is now at
most 200 characters, checked where one is made or renamed.

docs/wiki/Spaces-and-Documents.md

@@ -11,7 +11,8 @@ uni do not get mixed up.
 - `w` shows every space at once, with what was last written in each. `hjkl`
   move, `Enter` goes there.
 - `:space! <name>` makes a space, `:name! <name>` renames the one you are in,
-  and `:rmspace` puts it in the trash with everything in it.
+  and `:rmspace` puts it in the trash with everything in it. Two spaces cannot
+  share a name, whatever the case.
 - `:set noboard` turns the board off for a space, `:set board` back on.
 
 ![Every space at a glance](../images/overview.png)