A shared value belongs to the run, so a declaration carries its scope (#91)

A variable or a state machine declared `global` keeps one value for the whole
run instead of one per behavior instance. The scope sits on the declaration --
`PropertyDef.scope` and `StateMachineAsset.scope`, both optional, absence
meaning `local` -- and `local` is never written, so every project made before
this saves back unchanged.

**The name is the key.** Two behaviors declaring a global `score` mean one
variable; a behavior declaring `score` locally keeps its own. Kinds that
disagree about one global name stop the run before it starts, with the two
behaviors named; starting values that disagree are a warning on the behavior
that declares it, and the first declaration wins (`globalConflicts`).

The runtime needs no redirection layer. A global variable falls out of
`schemaForObject`, so it never reaches `entity.props` -- and then the whole rule
is "whoever declares the name takes their own value" (`graph/sharedState.ts`).
The `Variables` node is untouched and stays transparent: switching a variable to
global breaks no existing graph, because `GraphInterpreter.readAll` builds a
fresh object per read anyway and only asks `port.key in all`.

That same cut answers the inspector. A shared value has no owner, so the placed
object shows no row for it and can hold no override -- `SetVariableScopeCommand`
takes the deviations that were there and undo gives them back.

Both places say it in words rather than leaving a glyph to be guessed: the
declaration explains the mark below the list, and the placed object names the
variable it no longer holds instead of claiming the behavior declares none --
which is what it said before, untruthfully. Screenshots of the e2e run turned
that up; no check had been looking at it.

`SharedState` hangs off `NodeContext`, is built in `BehaviorSystem.start()` from
the *declarations* rather than from any instance (a global variable exists
before its behavior is placed), and is thrown away with the run: nothing is
written back, not even in the editor run. `GraphInterpreter.call` passes it on
through `...ctx` while nested calls keep getting fresh local variables, which is
why `program.defaults` is stripped of global keys.

Two traps this hit. `defSignature`/`assetSignature` have to carry the scope or
the inspector does not rebuild after the switch -- and a state machine has no
rows at all, so its scope is appended to the signature by hand. And the object
inspector does not go through `schemaForObject` for behaviors, it reads
`asset.variables` directly; filtering in one place was not enough, and no unit
test covered that path. The e2e caught it (`global: true` where false belonged),
and the rule now lives in one named function, `objectRows`.

Scopes `object` and `scene` are deliberately not built. The object case already
has a route (`Object by name` -> `Properties`), and a run knows exactly one
scene, so `scene` would be indistinguishable from `global` until #39 exists.

Closes #59: a machine's scope is a word on the machine asset, not a setting per
`Set state` node, so the same machine cannot live in two places at once.

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

src/runtime/systems/BehaviorSystem.ts

@@ -1,11 +1,15 @@
 import {
   asNumber,
+  buildSharedState,
+  emptySharedState,
+  globalConflicts,
   type EntityHandle,
   type Graph,
   type GraphNode,
   GraphInterpreter,
   isHandleEnabled,
   type NodeContext,
+  type SharedState,
   type TraceListener,
   type Value,
 } from '../../core/graph';
@@ -83,6 +87,7 @@ export function behaviorError(world: World): string | undefined {
 export class BehaviorSystem implements System {
   private instances: Instance[] = [];
   private world: World | null = null;
+  private sharedState: SharedState = emptySharedState();
   private readonly collisions: CollisionSystem | null;
   private readonly animations: AnimationSystem | null;
 
@@ -100,9 +105,24 @@ export class BehaviorSystem implements System {
     this.animations = sources.animations ?? null;
   }
 
+  /** The values `global` declarations share this run – read-only to callers, rebuilt by `start`. */
+  get shared(): Readonly<SharedState> {
+    return this.sharedState;
+  }
+
   start(world: World): void {
     this.world = world;
     this.instances = [];
+    // Built fresh for every run: a shared variable starts at its declared value each time Play does.
+    this.sharedState = buildSharedState(this.project);
+    // Two behaviors declaring one global name with different kinds: whichever ran second would read
+    // a value of a type it never declared, so nothing runs until that is settled.
+    const blocking = globalConflicts(this.project).filter((c) => c.severity === 'error');
+    if (blocking.length > 0) {
+      for (const conflict of blocking) world.log.push(`${ERROR_PREFIX}: ${conflict.message}`);
+      world.stopRequested = true;
+      return;
+    }
     this.syncInstances(world);
   }
 
@@ -341,6 +361,7 @@ export class BehaviorSystem implements System {
       owner: entity.parent,
       variables: entity.props,
       states,
+      shared: this.sharedState,
       time: world.time,
       dt,
       findByName: (name) => world.findByName(name) ?? null,