Let a list hold structs, and read or write variables in bundles

A list item may now be a struct. The depth is bounded in the declaration
rather than by a comment: FieldKind (number|text|boolean|color) says what a
struct field holds, ListItemKind adds struct on top and says what a list
holds, so list -> struct -> primitive is the deepest storable shape and
sameValue stays finite. A struct item renders inline in the List tab, which
is why it needs no path in the slot address.

Four paths silently assumed a list item was a primitive: sameValue compared
items with ===, textOf would have printed [object Object], asListItem turned
a struct into text on Set item and Fill, and Index of used indexOf, which is
reference equality and could never have found a struct.

Tetris now runs on bundles. shapes is 112 {x, y} structs instead of 224 loose
numbers, piece and the candidate are structs, so Next move hands back a whole
candidate and each strand packs only the field it changes, since mergeStruct
leaves the rest standing. Declared behavior ports drop 49 -> 35, the master's
variables 18 -> 13, its graph 84/94 -> 69/76 nodes/edges.

Get variables and Set variables declare their ports from a list of variable
names, the way Pack and Expand already do, so one node can read or write
several at once. Renaming a variable rewires them, because there the port key
is the variable name.

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

src/core/graph/values.ts

@@ -39,16 +39,21 @@ export function asList(value: Value | undefined): ListValue {
   return [value];
 }
 
-/** A value narrowed to what a list may hold: handles and nested lists have no stored form. */
-export function asListItem(value: Value | undefined): number | string | boolean {
-  if (typeof value === 'number' || typeof value === 'boolean' || typeof value === 'string') return value;
-  if (value === null || value === undefined) return 0;
-  return asString(value);
+/** A value narrowed to what a list may hold: a struct stays one, handles and lists flatten. */
+export function asListItem(value: Value | undefined): number | string | boolean | StructValue {
+  if (isStructValue(value)) return value;
+  return asPrimitive(value);
 }
 
 /** A value narrowed to what a struct field may hold: a list stays a list, anything deeper flattens. */
 export function asField(value: Value | undefined): FieldValue {
-  return isList(value) ? value : asListItem(value);
+  return isList(value) ? value : asPrimitive(value);
+}
+
+function asPrimitive(value: Value | undefined): number | string | boolean {
+  if (typeof value === 'number' || typeof value === 'boolean' || typeof value === 'string') return value;
+  if (value === null || value === undefined) return 0;
+  return asString(value);
 }
 
 export function asBoolean(value: Value | undefined): boolean {