Define a struct once as an asset and reuse its shape (#253)

A struct used to declare its fields wherever it stood. The same shape
in two variables meant typing it twice, and Pack/Expand copied it once
without staying in sync. A Struct asset now holds the shape in one
place, and every struct that picks it as its type follows it.

- New asset type "Struct" (not placeable, group Structs, own icon).
  The inspector shows its fields with name, kind and default value,
  and "Open" shows them in the variables tab.
- Struct variables and struct fields get a Type row: own fields, any
  Struct asset, or "Make struct type". With a type the fields come
  from the asset and cannot be added, renamed or removed there. The
  values stay per variable, overrides on placed objects still work.
- Pack and Expand get the setting Type. With a type their ports are
  the fields of the asset and read-only in the inspector.
- A change to the asset reaches every typed place in one undo step
  (syncStructTypes). A new field arrives with its default. A removed
  field goes along with its port, edge and typed value. A renamed
  field keeps its value in variables, overrides and on ports with
  edges, also through nested types.
- Undo for these edits records plain assignments (Recorder), because
  the reach of a change is not known in advance.
- Cycles such as Loot in Loot, also through another type, are refused
  and not offered in the picker.
- Deleting the asset names how many places use it. Afterwards each
  place keeps its fields as its own.
- Loading reshapes a stale copy in a project file to its type.
- "Make struct type" creates an asset from an existing struct and
  links the place to it.

Differs from the plan:

- A changed default value applies to new fields and newly typed
  places only. Existing values stay.
- Removing a single field shows no warning. Undo brings it back.

Typed lists are left for #254. Lists stay dynamic for now.

Found by e2e: after deleting or renaming the type the inspector did
not rebuild and kept showing the old name. Fixed, the check was red
before.

Tests:
- ./check.sh: 182 files, 2661 tests, CHECK EXIT 0
- GM_STEPS=1,4,21,22,54: ALL PASSED, 176 checks
- 14 unit tests in tests/core/struct-types.test.ts
- New e2e module 54 with 14 checks, real mouse. Module 04 counts
  23 asset types.
- Counter-probes: without the rename pass 4 unit tests fail, without
  reshaping values 8, without the cycle rule 1. In the browser,
  without spreading changes and with + Field despite a type, 3
  checks fail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tools/e2e/index.mjs

@@ -56,6 +56,7 @@ import { snapshots } from './50-snapshots.mjs';
 import { clipboard } from './51-clipboard.mjs';
 import { hierarchyGroups } from './52-hierarchy-groups.mjs';
 import { curves } from './53-curves.mjs';
+import { structTypes } from './54-struct-types.mjs';
 
 export const STEPS = [
   hierarchy,
@@ -111,4 +112,5 @@ export const STEPS = [
   clipboard,
   hierarchyGroups,
   curves,
+  structTypes,
 ];