Drag cards out into columns, and columns among and into columns

The user wants to drag a card out between columns to make a new column,
and to drag columns to reorder them or put them inside other columns.

Their pick for where: the row of titles, the gaps between columns and
the space past the last one are among the columns. Something dragged
there becomes a column, before the column under the mouse on its left
half and after it on its right half, after the one to the left in a gap.
Anywhere else it goes into the column under the mouse by the card rules,
so a column dragged down into another one becomes a card, its own cards
its block. A column's title no longer means "its first card"; the top
half of its first card does that. The root's column stays first and
can't be dragged (the user's pick): it says so.

The board holds still around a drag. Besides the kept widths, the room a
dragged column took stays empty while it isn't a column (Drag.room):
without it, the columns to its right slid left under the mouse each
time it stopped being one, and the card landed a column too far.
Dropping it back in that room makes it that column again.

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

README.md

@@ -100,7 +100,7 @@ The plan for the everyday keys (all of them are in DESIGN.md):
 | `<Space>o`        | this branch out, into a document of its own, with a portal in its place (`:branchout`) |
 | Enter or `gf` `gu` | through a portal / up to the portals that lead here |
 | `<Space>`         | the action menu: `c` color, `t` tags, `b` documents, `i` inbox, `vo` outline, `vn` note, `vb` board… |
-| board             | `<Space>vb` or `:board`: a column per node without a parent, its children as cards, a card with children wrapping them as Scratch's loop blocks do; `j` `k` cards, `h` `l` columns, `M` then `hjkl` (or a drag) moves a card |
+| board             | `<Space>vb` or `:board`: a column per node without a parent, its children as cards, a card with children wrapping them as Scratch's loop blocks do; `j` `k` cards, `h` `l` columns, `M` then `hjkl` (or a drag) moves a card; drag a card onto the titles' row or into a gap to make it a column, and a column by its title to reorder it or put it in another column |
 | `Ctrl-w v` `Ctrl-w s` | split the pane side by side / stacked (`Ctrl-w hjkl` to move between them) |
 | `gt` `gT`         | the next / previous document                       |
 | `:` `?`           | command line, help (search it with `/`, Enter runs the key or command) |