Edit the graph during play and watch values and flow run through it

The graph was only locked by five edit-mode guards in GraphCanvas. The
running game reads the graph by reference each frame, so a new node, edge,
changed value, disconnect or undo takes effect from the next frame. Stop
discards these edits like everything else changed in play. A new On start
node does not fire for a behavior that has already started.

During play every data output shows the value it last produced, in the
highlight colour and refreshed ten times a second. Values are kept per
graph, because node ids are only unique within one; with several
instances the most recent value wins. The Variables node reads the live
variable bag, so only its own outputs are kept, as a snapshot.

Each time an exec edge fires, a bead runs along it in 0.3 s, at most one
every 0.15 s per edge, following the same curve the edge is drawn with.
Beads run on game time and freeze while paused; the game logic is
unchanged.

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

tools/lib/harness.mjs

@@ -276,7 +276,7 @@ function makeTools({ cdp, projectId, baseUrl, consoleLog, check, shotPrefix }) {
   const storedObjects = () => json('__gm.state.activeScene.objects');
   const asset = (id) => json(`__gm.state.project.assets.find((a) => a.id === ${JSON.stringify(id)})`);
   const world = () => json('__gm.world()');
-  const entity = async (objectId) => (await world()).entities.find((e) => e.objectId === objectId);
+  const entity = async (objectId) => (await world())?.entities.find((e) => e.objectId === objectId);
   const select = (selection) => evaluate(`__gm.state.select(${JSON.stringify(selection)})`);
   /**
    * Runs `body` (JavaScript source) against the page's model and previews the result, the way a