docs: a list is the fourth append-only shape (V-453)

docs/design.md

@@ -630,6 +630,31 @@ add a new principle; it applied the existing one at smaller and smaller scope.
 
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+## A list is the fourth shape
+
+Facts, notes and tasks were the three append-only shapes. `list_items` is the
+fourth (Vikunja #453): an item, a status, and a list tag.
+
+It is not a task. Milk is not work, nothing prioritises it, and the ranker must
+not start counting groceries as outstanding errands. It is not a fact either,
+because it claims nothing about the world. What it is, is a set that grows and
+shrinks.
+
+The property that makes the separate table worth it: no predicate reads a list.
+Nothing ranks it, nothing nudges about it, the digestion worker ignores it. So
+two people adding to the same list at once cost nothing — there is no order to
+disagree about and no lifecycle past crossed-off.
+
+The unique index is the tasks one, per list, and live rows only. Saying "молоко"
+twice before the shop is one line; saying it again next week, after the last one
+was crossed off, is a new line.
+
+Spoken, it is four turns: add, read back, cross one item off, cross the lot off.
+All four are matched deterministically in `internal/router/list.go` and all four
+run at stage 0, because an add and a read-back are cheap and should not depend on
+the resident model having a good turn. Crossing one item off claims the turn only
+when the list holds that item, which is what keeps "купил новый ноутбук" a note.
+
 ## Calendar
 
 Integration with **Radicale** (self-hosted CalDAV), not Nextcloud. Scope is