Keep behavior variables private to their own graph (#257)

A behavior often needs working state such as a timer or a counter.
Until now every variable showed up on each placed object and in the
Properties menu, so the inspector filled up with values nobody should
touch. A private variable works like any other inside its graph but
is not offered anywhere outside.

- Each behavior variable gets a lock switch next to Keep. It starts
  off, so existing projects stay as they are (`privateVariables`).
- A placed object shows no private variable, neither in the
  inspector nor while the game runs. If all are private, it says so.
- Switching a variable to private drops its overrides on every
  placed object in one undo step. Switching back does not restore
  them.
- A running game does not write a private value back as an
  override, and a project change during the run does not reset it.
- `+ Property` no longer offers private variables. A port that
  names one by hand still reads and writes it without a warning.
- The default value of a private variable is read-only in the asset
  inspector. Its shape (name, fields, type, delete) stays editable.
- Keep is now a pin icon instead of a word. With the lock added, the
  header of a list variable had no room left and the lock covered
  `+ Item`.

Not done: the animation track picker still offers private variables.

Tests: ./check.sh -> 186 files, 2711 tests, CHECK EXIT 0.
e2e 1,19,20,21,56 -> ALL PASSED, 119 checks. New module 56 has
7 checks with a real mouse. Counter-checks fail as expected: unit
without the write-back filter, the sync guard, dropping overrides or
loading the flag. Browser with all variables on the object, an
unfiltered menu, or no value lock.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/core/model/Asset.ts

@@ -37,14 +37,20 @@ export interface SceneAsset extends AssetBase {
  * `keptVariables` names the variables a `Run behavior` call hands on to the next call from the same
  * node on the same object (#229); the others start at their default every call. Absent keeps none,
  * so a project from before the switch runs as it did, and a new variable starts out not kept.
+ *
+ * `privateVariables` names the variables only the behavior's own graph sees (#257): a placed object
+ * neither shows nor overrides them, and a `Properties` node does not offer them. Absent is none.
  */
 export interface BehaviorAsset extends AssetBase {
   type: 'behavior';
   variables: PropertyDef[];
   keptVariables?: string[];
+  privateVariables?: string[];
   graph: Graph;
 }
 
+type VariableSwitch = 'keptVariables' | 'privateVariables';
+
 /** Whether a call of this behavior hands `key` on to the next call (#229). */
 export function isKeptVariable(asset: BehaviorAsset, key: string): boolean {
   return asset.keptVariables?.includes(key) === true;
@@ -52,10 +58,28 @@ export function isKeptVariable(asset: BehaviorAsset, key: string): boolean {
 
 /** Writes the switch for one variable; an empty list is dropped rather than stored. */
 export function setKeptVariable(asset: BehaviorAsset, key: string, on: boolean): void {
-  const rest = (asset.keptVariables ?? []).filter((k) => k !== key);
+  setSwitch(asset, 'keptVariables', key, on);
+}
+
+/** Whether `key` is hidden from everything outside the behavior's own graph (#257). */
+export function isPrivateVariable(asset: Asset, key: string): boolean {
+  return asset.type === 'behavior' && asset.privateVariables?.includes(key) === true;
+}
+
+export function setPrivateVariable(asset: BehaviorAsset, key: string, on: boolean): void {
+  setSwitch(asset, 'privateVariables', key, on);
+}
+
+/** The variables a placed object shows and overrides: all but the private ones. */
+export function publicVariables(asset: BehaviorAsset): PropertyDef[] {
+  return asset.variables.filter((def) => !isPrivateVariable(asset, def.key));
+}
+
+function setSwitch(asset: BehaviorAsset, list: VariableSwitch, key: string, on: boolean): void {
+  const rest = (asset[list] ?? []).filter((k) => k !== key);
   const next = on ? [...rest, key] : rest;
-  if (next.length > 0) asset.keptVariables = next;
-  else delete asset.keptVariables;
+  if (next.length > 0) asset[list] = next;
+  else delete asset[list];
 }
 
 /**