Add the board, and finish v1.1 (stage 9e)

The board is the user's design: the document as lists and blocks, the
way Scratch nests blocks inside a loop. Each node without a parent (the
root, then each loose node) is a column with its title in the top border;
its children are cards in it, one under the other; a card with children
on show wraps them in a C: a joint at its bottom left, a bracket down the
left two cells further in, and a foot closing it. Blocks nest as deep as
the tree does. The columns by tag or color the first design had are
dropped (the user's pick), and :board takes no arguments any more.

j and k go down and up a column in reading order, into the blocks; h and
l go to the column beside, onto the card nearest the cursor's height.
Titles are typed right in their cards, and visual mode selects in
reading order as the outline does. <Space>vb or :board shows it.

Move mode (M, the user's pick) moves the card through the tree: h and l
to the end of the column beside, j and k past the cards around it, into
an open block and out of one at its end. A column's own node trades
places with the loose column beside it; the root's column stays first.
A drag does the same with the mouse: dropped on a card it goes after it,
on a block's card into it, on a column at its end.

The fallback message for a key a view ignores now names the view ("does
nothing in the board view"). The version goes to 1.1.0: stage 9 is done.

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

Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "cumulus"
-version = "1.0.0"
+version = "1.1.0"
 edition = "2024"
 # The version cumulus is built and tested with (pinned in rust-toolchain.toml).
 rust-version = "1.90"