A game that is more than one scene needs a way from one to the next, so the graph can load one (#39)

Load scene sets a flag on the World like Stop and Restart do; the swap
happens after the tick, and the last wish of a tick wins. Exactly one
thing crosses over: the SharedState, so global variables and global
machines keep what they know while everything local is built anew.
Loading the scene that is already running rebuilds it, which is what
"reset the level" means when the score must survive.

A run starts either from the start scene (a new project field, set from
the asset panel and marked with an arrow in the list) or from whatever
scene is open. The toggle next to Play says which question is asked and
remembers the answer in localStorage: it is a way of working, not part
of the game.

Sounds no longer end because a scene ends. A voice belongs to the
behavior that started it and stops right after On destroy, so the scene
swap is no longer a special case at all. For the enemy that plays its
explosion and destroys itself in the same breath there is now a
detached input on Play sound.

The scene selector stays usable while a run plays, because a double
click in the asset panel already did the same thing unguarded. It moves
what the editor looks at, not what the run plays.

src/core/graph/NodeContext.ts

@@ -125,10 +125,20 @@ export interface NodeContext {
    * because the asset says what a sound *is* and the node only colours it. Every call is a voice of
    * its own: two plays overlap, whether they loop or not, which is what makes one sound effect
    * fired twice in a row audible twice.
+   *
+   * The voice belongs to the behavior that started it and ends when that behavior is destroyed –
+   * a sound is something a thing in the world is doing. `detached` gives that up on purpose, for
+   * the one case where the rule is in the way: the death sound of an object that destroys itself.
    */
-  playSound(assetId: string, volume: number, loop: boolean | undefined): void;
+  playSound(assetId: string, volume: number, loop: boolean | undefined, detached: boolean): void;
   /** Silences every voice of that sound; `''` silences everything this run started. */
   stopSound(assetId: string): void;
+  /**
+   * Asks for another scene, by asset id. The swap happens once the frame is over – a graph must be
+   * able to finish the flow it is in, and half a frame in the old world and half in the new one is
+   * the kind of thing nobody can debug. The last request of a frame is the one that happens.
+   */
+  loadScene(sceneId: string): void;
   /**
    * The position is local to `parent` (world when none); null when the asset does not exist or
    * cannot be placed. A spawned behavior runs its `On start` on the next tick.