feat(todo): tasks dragged into place or under another, the add button dropped where the task goes (ADR 0033, D16)

- A project's row, held, is dragged before or after another, or a
  little to the right to become its sub-task, or onto a section's
  heading; its sub-tasks go with it, and Order changed offers Undo
  (TaskRepository.place). The list scrolls at its edges.
- The add button is dragged onto a project's list to add the task
  between two rows, onto a day in Upcoming for that day, onto Today
  for today; dropped back on its place, an ×, it gives up.
- Holding a row now drags it; a swipe to the right selects it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0033-halkyon-todo.md

@@ -192,7 +192,7 @@ alternative was put to them, the choice is theirs.
   the handle dragged up folding it; a button beside the month goes back to today once
   another week shows. Tasks are chosen to act on together, as Todoist's: a row swiped
   right or held, or *Select tasks* in Today's, Upcoming's and a project's ⋮, a tap then
-  choosing or letting go; the app bar counts them, with an arrow (or back) ending it,
+  choosing or letting go (a row held is dragged since D16); the app bar counts them, with an arrow (or back) ending it,
   and a bar takes the navigation bar's place: date, move, labels (those all have
   ticked; a tick added to each, one unticked taken off each), priority, and ⋮ with
   complete (undone together), duplicate, deadline and delete (asked first). *Assign*
@@ -221,10 +221,22 @@ alternative was put to them, the choice is theirs.
   chips once typing starts, a chip picked taking its token out of the title, and the
   description and the deadline behind its + (the description also from the down
   arrow).
-- **To come, in this order**: rows dragged into another order, and a little to the
-  right to become the sub-task of the row above, with Undo; the add button dragged onto
-  the list to add a task there, dropped back on its place (an ×) to give up, wherever
-  it shows; suggestions of projects and labels as `#` and `@` are typed.
+- **D16 Dragging** (the developer's, 1 Nov, as Todoist's): a row of a project's list
+  (the Inbox's too), held, is dragged into another place, a line showing where: the
+  upper half of a row puts it before that row, the lower half after it, and the lower
+  half reached 48 px to the right of where the drag began makes it that row's last
+  sub-task; a section's heading takes it first in the section, or last in the group
+  before from its upper half. It goes with its sub-tasks, and *Order changed* offers
+  to undo it (`TaskRepository.place`, never under itself or its own sub-task). The
+  list scrolls at its edges. Holding a row therefore drags it everywhere it can be
+  dragged and no longer selects it (D14): a swipe to the right does. The add button
+  is dragged straight away, without a hold, its place meanwhile a larger × where
+  dropping it gives up: dropped between a project's rows, quick add adds there (each
+  next task below the one before); on a day in Upcoming, for that day; on Today, for
+  today. Today's and Upcoming's rows aren't dragged, their order being their dates',
+  times' and priorities'.
+- **To come**: suggestions of projects and labels as `#` and `@` are typed; dragging a
+  task onto another day in Upcoming.
 
 ## Steps