Variables and state machines can reset with every scene (#164)

A shared variable or machine used to live either per object or for the
whole run. Levels need a third lifetime: shared like global, but built
fresh from the declarations whenever a scene is loaded.

- New scope `scene` for Variables assets and state machines. A missing
  field keeps the old meaning: `global` for assets, `local` for
  machines, so existing projects do not change.
- Returning to a scene that was visited before is fresh as well. There
  is no memory per scene.
- A `scene` machine announces its start state again after a scene swap,
  so `On state entered` in the new scene hears it. A global machine
  stays quiet because it never left its state.
- The rule lives in two pure functions in `sharedState.ts`
  (`carriedForward`, `restartedAssets`). BehaviorSystem only calls them.
- The inspector shows a Scope dropdown on Variables assets and a third
  step on the machine dropdown, with a note on what `scene` means.

Decisions:
- `scene` and `persistent` exclude each other (variant A). The choice
  dialog was cancelled, so this is an assumption to revisit. A
  scene-scoped asset hides the Persistent switch, and switching to
  `scene` drops the flag (undo restores it). The guard sits in
  `isPersistent` itself, so no path can save values the scope throws
  away anyway.

Findings:
- `clickText('Scene')` hits the Scene tab, not the menu entry. One e2e
  check was red because of that, and its partner was falsely green
  because it compared `undefined` with `undefined`. Both use `clickLast`
  now.

Tests:
- ./check.sh: 144 files, 2142 tests, CHECK EXIT 0 (before 143 / 2133)
- 9 new unit tests in tests/core/scene-scope.test.ts
- e2e 32 fresh: ALL PASSED, 17 checks
- e2e 1-3,17,18,26,32,35 fresh: ALL PASSED, 213 checks
- 3 new e2e checks (35: scene-scoped asset resets on swap, 32h: dropdown,
  note, hidden Persistent switch)
- Counter-checks bite: ignoring `restarted` fails 2 unit tests and the
  new check in 35, the old announcement rule fails 1, dropping the scope
  guard in `isPersistent` fails 1. Originals restored, cmp IDENTICAL.
- Check sum 1307 (1304 + 3) is computed. Last CI measurement: 1293.

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

src/core/store/serialize.ts

@@ -53,7 +53,7 @@ import {
   type Vec2,
 } from '../model/Properties';
 import { PROJECT_FORMAT_VERSION, type Project } from '../model/Project';
-import { readScope } from '../model/Scope';
+import { readScope, readSharedScope } from '../model/Scope';
 import type { SceneObject } from '../model/SceneObject';
 import {
   DEFAULT_FILL,
@@ -171,7 +171,9 @@ export function deserializeProject(raw: unknown, fallbackId = ''): Project {
       });
     } else if (type === 'variables') {
       const shared: VariablesAsset = { id, type, name, props, variables: variables.get(id) ?? [] };
-      if (a.persistent === true) shared.persistent = true;
+      // Scope first: a `scene` asset is never persistent, so the switch that cannot apply is dropped.
+      if (readSharedScope(a.scope) === 'scene') shared.scope = 'scene';
+      else if (a.persistent === true) shared.persistent = true;
       assets.push(shared);
     } else if (type === 'statemachine') {
       const machine: StateMachineAsset = {
@@ -181,7 +183,8 @@ export function deserializeProject(raw: unknown, fallbackId = ''): Project {
         props,
         graph: normalizeGraph(a.graph, (def) => def.type === STATE),
       };
-      if (readScope(a.scope) === 'global') machine.scope = 'global';
+      const scope = readScope(a.scope);
+      if (scope !== 'local') machine.scope = scope;
       // Kept as written even where it names no state any more: the same as a deleted start.
       if (typeof a.start === 'string' && a.start !== '') machine.start = a.start;
       assets.push(machine);