Let a variable declare struct fields

The value type was already there: StructValue, textOf, sameValue,
writeTyped/mergeStruct and the struct port type came with the first
part, and Get/Set variable carry any-ports, so a bundle already flowed
through the graph unchanged. What was missing was the declaration.

PropertyDef gains kind 'struct' with a list of fields, VARIABLE_KINDS a
fifth kind, variableDef a fifth parameter. A declared field is a
primitive - a list field would need a nested editor and therefore the
path in the slot address, the same step "a list of structs" is waiting
for. coerceValue now returns PropertyValue rather than FieldValue, the
first value type that is not a field; none of its five callers break.

In the inspector a struct variable is a block, not a tab. The list tab
exists because hundreds of rows fit in no inspector row; a struct is the
opposite case - few fields that fit under each other. It is the port
editor Pack already has, one level down and with a value column: type,
key and default value in one place. A placed object sees the same fields
as plain value rows: overriding yes, declaring no.

Adding or removing a field touches the declaration, the default, the
behavior's value and every instance override in one command, so a stored
value can never keep a shape its declaration no longer promises. The row
handlers read the bundle at the moment of the change rather than closing
over a snapshot - a field writes the whole bundle, so with a snapshot
the second edited row revives the first one's old value. defSignature
carries the fields, otherwise a new row appears only at the next
rebuild.

Fixes a fixture fault this uncovered: step 10 pushed its text object
into the scene without instanceEdits, which overrideSites walks with
Object.entries. AddStructFieldCommand is the first caller to reach it
from that state; the step was green alone and failed three checks in the
full suite.

Moves the dynamic-ports note to done - all three of its parts are built.

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

src/core/store/serialize.ts

@@ -27,6 +27,7 @@ import {
   type PropertyDef,
   type PropertySchema,
   schemaFor,
+  type StructFieldDef,
   VARIABLE_KINDS,
   variableDef,
   type VariableKind,
@@ -176,13 +177,28 @@ function normalizeVariables(raw: unknown): PropertyDef[] {
     const items = (LIST_ITEM_KINDS as readonly string[]).includes(String(v.items))
       ? (v.items as ListItemKind)
       : undefined;
-    const def = variableDef(key, kind, undefined, items);
+    const def = variableDef(key, kind, undefined, items, normalizeStructFields(v.fields));
     def.default = coerceValue(def, (v.default ?? def.default) as PropertyValue);
     defs.push(def);
   }
   return defs;
 }
 
+/** An unreadable field is dropped rather than defaulted: an invented key would claim a value. */
+function normalizeStructFields(raw: unknown): StructFieldDef[] {
+  const fields: StructFieldDef[] = [];
+  const seen = new Set<string>();
+  for (const item of Array.isArray(raw) ? raw : []) {
+    const f = asRecord(item);
+    const key = typeof f.key === 'string' ? f.key : '';
+    if (!/^[A-Za-z_][A-Za-z0-9_]*$/.test(key) || seen.has(key)) continue;
+    if (!(LIST_ITEM_KINDS as readonly string[]).includes(String(f.kind))) continue;
+    seen.add(key);
+    fields.push({ key, kind: f.kind as ListItemKind });
+  }
+  return fields;
+}
+
 function normalizePorts(raw: unknown): DeclaredPort[] {
   const ports: DeclaredPort[] = [];
   const seen = new Set<string>();