Drop the per-list item declaration; every item carries its own kind

A list no longer says what its items are. ListDef.items, ListDef.fields,
ListFieldDef.items and SetListItemsCommand are gone; the single seam is
src/core/properties/listItems.ts. JSON already tells number, text, boolean
and struct apart, so only two kinds need a tag: {"$color": n} and the new
{"$texture": id}. Field names cannot start with "$", so a tag never
collides with a struct field. On a data link a texture item arrives as
its bare id; the graph has no images.

List tab: the mark in front of each row is the kind picker, "+ Item"
asks for the kind, both are ordinary value edits (one undo step). A list
inside a struct field only offers the flat kinds because the depth rule
forbids a struct item there (ListAccess.kinds); the menu used to offer
it and silently do nothing.

Legacy lists that declared items: 'color' are tagged on load.

Actor demo: the four walk animations and four standing frames live in
two list variables of texture items indexed by facing (Item at), so the
graph can be rebuilt in the editor. The stand frames get their grid ids
back; 44 nodes instead of 46.

Also: the user's edits to on-start-during-play.md and the new note
reseed-because-of-list.md.

Verified: check.sh 794 tests, full e2e 631 checks, smokes 24/19/20.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

tests/core/struct-variable.test.ts

@@ -106,7 +106,7 @@ describe('a variable of type struct', () => {
   });
 
   it('coerces a stored value to the shape its declaration promises', () => {
-    const def = variableDef('piece', 'struct', undefined, undefined, [
+    const def = variableDef('piece', 'struct', undefined, [
       { key: 'x', kind: 'number' },
       { key: 'on', kind: 'boolean' },
     ]);