Let each variable of a behavior choose whether calls keep it (#229)

The first version of #229 put a Keep state switch on the Run behavior
node. The user wanted it where the variable is declared instead. Whether
a value lives on belongs to the variable, not to every call site.

- The node switch keepState is gone again. RunBehavior, CallResult and
  nested.ts are back to what they were before #229.
- BehaviorAsset.keptVariables lists the kept names. It lives on the
  asset and not in PropertyDef, because that is the schema of every
  inspector row.
- A variable added from now on starts with Keep on. Variables in
  existing projects start with Keep off, so old projects run unchanged.
- SetVariableKeptCommand flips the switch as one undo step. Add, remove
  and rename carry the list along. Loading drops names that are not
  declared.
- The interpreter keeps only the listed variables, per Run behavior
  node and per object as before. The others start at their defaults.
- Local state machines in a called behavior start empty on every call
  again, as before #229. Global and scene machines are unchanged.
- The inspector shows a Keep button in each variable row of a behavior.
  Its state is part of the asset signature, so a click redraws the row.
  Without that the button kept its old look until the inspector was
  reopened.

Found: the project Garden of Sacrifices still has keepState = true on
the call Player -> Character Movement. It is ignored now. Direction
there predates the switch, so Keep is off until set by hand.

Tests:
- ./check.sh: 163 files, 2392 tests, CHECK EXIT 0 (before 2388)
- tests/core/keep-state.test.ts rewritten, 15 tests
- e2e 19: ALL PASSED, 35 checks. 19c-ii now clicks the Keep button
  of a new variable: on when added, off by a click, on again, off
  after undo.
- Counter-checks, each fails its own test: keep list ignored (2),
  states kept (1), new variable not kept (8), rename drops the list
  (1), no Keep button (e2e 19c-ii).

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

tests/core/node-catalog.test.ts

@@ -137,18 +137,12 @@ const CASES: Record<string, Case[]> = {
       name: 'asks for the named behavior to run, passing its inputs on',
       inputs: { power: 3 },
       params: { behavior: 'b1' },
-      call: { behaviorId: 'b1', inputs: { power: 3 }, keep: false },
-      next: 'exec',
-    },
-    {
-      name: 'asks for the call to keep its state when Keep state is on',
-      params: { behavior: 'b1', keepState: true },
-      call: { behaviorId: 'b1', inputs: {}, keep: true },
+      call: { behaviorId: 'b1', inputs: { power: 3 } },
       next: 'exec',
     },
     {
       name: 'without a behavior chosen it still continues, calling nothing that exists',
-      call: { behaviorId: '', inputs: {}, keep: false },
+      call: { behaviorId: '', inputs: {} },
       next: 'exec',
     },
   ],