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/store/serialize.ts

@@ -184,9 +184,14 @@ export function deserializeProject(raw: unknown, fallbackId = ''): Project {
         graph: normalizeGraph(a.graph),
       };
       // Only names that are declared, each once, in declaration order.
-      const kept = new Set(Array.isArray(a.keptVariables) ? a.keptVariables : []);
-      const keptVariables = declared.map((v) => v.key).filter((key) => kept.has(key));
+      const declaredOf = (list: unknown): string[] => {
+        const named = new Set(Array.isArray(list) ? list : []);
+        return declared.map((v) => v.key).filter((key) => named.has(key));
+      };
+      const keptVariables = declaredOf(a.keptVariables);
       if (keptVariables.length > 0) behavior.keptVariables = keptVariables;
+      const privateVariables = declaredOf(a.privateVariables);
+      if (privateVariables.length > 0) behavior.privateVariables = privateVariables;
       assets.push(behavior);
     } else if (type === 'variables') {
       const shared: VariablesAsset = { id, type, name, props, variables: variables.get(id) ?? [] };