Start new behavior variables with Keep switched off

Keep is a choice the author makes per variable. Switching it on for
every new variable meant most behaviors carried state across runs that
nobody asked for, and the toggle had to be turned off by hand.

- AddVariableCommand no longer adds a new variable to keptVariables.
  Keep is only switched on by hand.
- Existing projects are unchanged. Their keptVariables stay as saved.
- The comment in Asset.ts describes the new default.
- The counter fixture in keep-state.test.ts now keeps count
  explicitly instead of relying on the old default.

Tests:
- ./check.sh: 2706 tests, CHECK EXIT 0
- e2e 1, 6, 19: ALL PASSED, 172 checks
- 19c-ii expects [] after adding, then ["var1"], [] and ["var1"]
  after undo, all clicked with the real mouse
- Before the fixtures were adjusted, 8 unit tests failed on the new
  default

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

src/core/commands/variableCommands.ts

@@ -25,7 +25,7 @@ import { requireBehavior, requireVariableOwner } from './require';
 
 /**
  * The variable's default value is written to `props` as well. On a behavior a new variable starts
- * out kept between calls (#229).
+ * at its default every call; "Keep" is switched on by hand (#229).
  */
 export class AddVariableCommand implements Command {
   readonly label: string;
@@ -47,14 +47,12 @@ export class AddVariableCommand implements Command {
       throw new CommandError(`Variable "${this.def.key}" already exists`);
     asset.variables.push(this.def);
     asset.props[this.def.key] = this.def.default;
-    if (asset.type === 'behavior') setKeptVariable(asset, this.def.key, true);
   }
 
   undo(project: Project): void {
     const asset = requireVariableOwner(project, this.assetId);
     asset.variables = asset.variables.filter((v) => v.key !== this.def.key);
     delete asset.props[this.def.key];
-    if (asset.type === 'behavior') setKeptVariable(asset, this.def.key, false);
   }
 }