todos: a backburner column, and the parked items on it

backburner joins TODO_STATUSES and the board's columns; the three items in the
backburner theme move onto the status, so parked is now a lane rather than a
folder you have to remember to open.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

todo/README.md

@@ -38,8 +38,9 @@ process everywhere, report which host is behind…
 
 - **`id`** — `<theme>/<slug>`, and it is the filename. Stable: rename the title freely, leave the id
   alone. Anything pointing at an item points at this.
-- **`status`** — `open` or `done` today. The board will want more (`doing`, `blocked`); adding one is a
-  new value here, not a new file or a new directory.
+- **`status`** — `open`, `doing`, `blocked`, `backburner` or `done`. The board (`/todos`) renders one
+  column per value; adding one is a new entry in `TODO_STATUSES` (`api/todos/todo-tree.ts`), not a new
+  file or directory. `backburner` is parked-not-dead — still counted as open by the index.
 - **`created`** — the date the item was raised. **`date_source`** says how much to trust it:
   `stated` (the prose named a date — true), `section` (inherited from its old section heading — close),
   `assumed` (unknown, floored at 2026-08-20, the day before the split — a bounded "before today"