Add the Variable node and show the variable name in node titles

Get/Set variable nodes now title themselves "Get <name>" / "Set <name>", and
the new Variable node just "<name>"; the title is the same folded or open,
and a collapsed node shrinks to it. The redundant "Variable: <name>" row is
gone unless the variable was deleted ("<name>?" is the only hint then).

The Variable node sets its value when an exec flow arrives and exposes the
value at any time. Its output is read when a downstream node pulls it, not
frozen at the write, so count = count + 1 is one Variable node plus an Add.
Get variable keeps reading once per pulse: switching it would change the
behaviour of existing games.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/core/graph/nodes/variable/GetVariable.ts

@@ -1,7 +1,6 @@
 import type { NodeDef } from '../../NodeRegistry';
-import { any, variableParam } from '../shared';
+import { any, readVariable, variableParam, variableTitle } from '../shared';
 
-import { textOf } from '../../../model/Properties';
 export const GetVariable: NodeDef = {
   type: 'GetVariable',
   label: 'Get variable',
@@ -10,7 +9,6 @@ export const GetVariable: NodeDef = {
   inputs: [],
   outputs: [any('value')],
   params: [variableParam],
-  run: (ctx, _inputs, params) => ({
-    outputs: { value: ctx.variables[textOf(params.variable ?? '')] ?? null },
-  }),
+  derivePorts: variableTitle('Get'),
+  run: (ctx, _inputs, params) => ({ outputs: readVariable(ctx, params) }),
 };

src/core/graph/nodes/variable/SetVariable.ts

@@ -1,8 +1,6 @@
 import type { NodeDef } from '../../NodeRegistry';
-import { writeTyped } from '../../values';
-import { any, exec, variableParam } from '../shared';
+import { any, exec, variableParam, variableTitle, writeVariable } from '../shared';
 
-import { textOf } from '../../../model/Properties';
 export const SetVariable: NodeDef = {
   type: 'SetVariable',
   label: 'Set variable',
@@ -11,9 +9,6 @@ export const SetVariable: NodeDef = {
   inputs: [exec(), any('value')],
   outputs: [exec()],
   params: [variableParam],
-  run: (ctx, inputs, params) => {
-    const key = textOf(params.variable ?? '');
-    if (key) writeTyped(ctx.variables, key, inputs.value);
-    return { next: 'exec' };
-  },
+  derivePorts: variableTitle('Set'),
+  run: writeVariable,
 };

src/core/graph/nodes/variable/Variable.ts

@@ -0,0 +1,18 @@
+import type { NodeDef } from '../../NodeRegistry';
+import { any, exec, readVariable, variableParam, variableTitle, writeVariable } from '../shared';
+
+/** Get and Set in one node: the output can be read at any time, an arriving flow writes the input. */
+export const Variable: NodeDef = {
+  type: 'Variable',
+  label: 'Variable',
+  category: 'variable',
+  description:
+    "Reads one of this behavior's variables at any time, and writes it when the flow arrives. " +
+    'A read after the write sees the new value.',
+  inputs: [exec(), any('value')],
+  outputs: [exec(), any('value')],
+  params: [variableParam],
+  derivePorts: variableTitle(''),
+  read: readVariable,
+  run: writeVariable,
+};