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/graph/savedValues.ts

@@ -1,6 +1,7 @@
 import type { VariablesAsset } from '../model/Asset';
 import type { PropertyBag, PropertyValue } from '../model/Properties';
 import type { Project } from '../model/Project';
+import { sharedScopeOf } from '../model/Scope';
 import { coerceValue } from '../properties/PropertySchema';
 
 /** What a save area holds: the values of each persistent Variables asset, keyed by asset id. */
@@ -15,8 +16,13 @@ export interface SaveArea {
   write(saved: SavedValues): void;
 }
 
+/**
+ * A `scene` asset is never persistent, whatever the flag says (#164): its values are thrown away
+ * with every scene, so there is nothing about them worth keeping between runs. Answered here rather
+ * than left to the commands, so no path can produce a saved value the scope would drop anyway.
+ */
 export function isPersistent(asset: VariablesAsset): boolean {
-  return asset.persistent === true;
+  return asset.persistent === true && sharedScopeOf(asset) !== 'scene';
 }
 
 function persistentAssets(project: Project): VariablesAsset[] {