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.

README.md

@@ -176,7 +176,8 @@ selected, and `:import` uploads a folder into the one being browsed
 (`:import!` takes a `.zip` instead). `:upload` outside the editor puts files
 into the folder being browsed, and so does dropping them on the list. `:name <text>` renames whatever is under
 the cursor — a document, a folder, or a card on a board — and `:name!` renames
-the space itself. The board has
+the space itself. Two spaces cannot share a name, in any case, and no name is
+longer than 200 characters. The board has
 `:board`, `:card <title>` (`:card!` links the open document to it),
 `:due <when|->`, `:repeat <rule|->`, `:done` and `:list <name>` (`:list!` deletes one).