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/core/graph/sharedState.ts

@@ -0,0 +1,115 @@
+import type { Asset } from '../model/Asset';
+import type { PropertyBag, PropertyValue } from '../model/Properties';
+import type { Project } from '../model/Project';
+import { isGlobal } from '../model/Scope';
+import type { PropertyDef } from '../properties/PropertySchema';
+import type { NodeContext, SharedState } from './NodeContext';
+
+export function emptySharedState(): SharedState {
+  return { variables: {}, states: {} };
+}
+
+/**
+ * Built once per run, from the declarations rather than from any instance: a global variable exists
+ * even before the behavior declaring it is placed. Two behaviors declaring the same name mean the
+ * same variable – the first declaration sets the starting value (see `globalConflicts`).
+ */
+export function buildSharedState(project: Project): SharedState {
+  const shared = emptySharedState();
+  for (const asset of project.assets) {
+    if (asset.type === 'behavior') {
+      for (const def of asset.variables)
+        if (isGlobal(def) && !(def.key in shared.variables))
+          shared.variables[def.key] = asset.props[def.key] ?? def.default;
+    } else if (asset.type === 'statemachine' && isGlobal(asset)) {
+      shared.states[asset.id] = '';
+    }
+  }
+  return shared;
+}
+
+/**
+ * Where two behaviors disagree about the same global name. A different kind is an error – one of the
+ * two would silently read a value of a type it never declared – while a different starting value is
+ * only a warning, since the first declaration decides and the run is well defined either way.
+ */
+export interface GlobalConflict {
+  key: string;
+  severity: 'error' | 'warning';
+  message: string;
+}
+
+export function globalConflicts(project: Project): GlobalConflict[] {
+  const first = new Map<string, { def: PropertyDef; asset: string; value: PropertyValue }>();
+  const conflicts: GlobalConflict[] = [];
+  const seen = new Set<string>();
+  for (const asset of project.assets) {
+    if (asset.type !== 'behavior') continue;
+    for (const def of asset.variables) {
+      if (!isGlobal(def)) continue;
+      const value = asset.props[def.key] ?? def.default;
+      const held = first.get(def.key);
+      if (!held) {
+        first.set(def.key, { def, asset: asset.name, value });
+        continue;
+      }
+      // One report per name: a third behavior disagreeing adds nothing the first line does not say.
+      if (seen.has(def.key)) continue;
+      if (held.def.kind !== def.kind) {
+        seen.add(def.key);
+        conflicts.push({
+          key: def.key,
+          severity: 'error',
+          message: `Global variable "${def.key}" is a ${held.def.kind} in ${held.asset} and a ${def.kind} in ${asset.name}.`,
+        });
+      } else if (!sameValue(held.value, value)) {
+        seen.add(def.key);
+        conflicts.push({
+          key: def.key,
+          severity: 'warning',
+          message: `Global variable "${def.key}" starts at ${display(held.value)} in ${held.asset} and at ${display(value)} in ${asset.name}; the first is used.`,
+        });
+      }
+    }
+  }
+  return conflicts;
+}
+
+/** Structs and lists compare by their stored shape, which is what the project file holds anyway. */
+function sameValue(a: PropertyValue | undefined, b: PropertyValue | undefined): boolean {
+  return a === b || JSON.stringify(a ?? null) === JSON.stringify(b ?? null);
+}
+
+function display(value: PropertyValue | undefined): string {
+  return typeof value === 'string' ? `"${value}"` : JSON.stringify(value ?? null);
+}
+
+/** The bag a variable is written to: its own when the behavior declares it, else the shared one. */
+export function variableBag(ctx: NodeContext, key: string): PropertyBag {
+  if (key in ctx.variables) return ctx.variables;
+  return key in ctx.shared.variables ? ctx.shared.variables : ctx.variables;
+}
+
+/** Everything the behavior can read under one name, its own declaration winning over a shared one. */
+export function readableVariables(ctx: NodeContext): PropertyBag {
+  const shared = ctx.shared.variables;
+  return isEmpty(shared) ? ctx.variables : { ...shared, ...ctx.variables };
+}
+
+/** Where a machine's current state is kept: shared when the machine is `global`, else per instance. */
+export function stateBag(ctx: NodeContext, machineId: string): Record<string, string> {
+  return machineId in ctx.shared.states ? ctx.shared.states : ctx.states;
+}
+
+/** The defaults a behavior instance starts with: a shared variable is not one of them. */
+export function withoutGlobals(defaults: PropertyBag, asset: Asset): PropertyBag {
+  if (asset.type !== 'behavior') return defaults;
+  const own = { ...defaults };
+  for (const def of asset.variables) if (isGlobal(def)) delete own[def.key];
+  return own;
+}
+
+function isEmpty(bag: PropertyBag): boolean {
+  for (const key in bag) if (Object.hasOwn(bag, key)) return false;
+  return true;
+}