refactor(react-doctor): extract isFunctionLikeVariableDeclarator to file scope (Bugbot #157 round 4)

The helper was inlined three times with identical bodies in
`noDerivedUseState`, `noPropCallbackInEffect`, and the newly added
`noMirrorPropEffect`. Bugbot pointed out the divergence risk (low
severity but a clear maintenance smell). Hoist to a single
file-scope const at the top of the module; the three rules now
share the implementation.

No behavior change. 511/511 tests still pass.

Co-authored-by: Aiden Bai <aidenybai@users.noreply.github.com>

Author: cursoragent

packages/react-doctor/src/plugin/rules/state-and-effects.ts

@@ -23,6 +23,16 @@ import {
 } from "../helpers.js";
 import type { EsTreeNode, Rule, RuleContext } from "../types.js";
 
+// HACK: shared by every rule that pushes an empty barrier frame on a
+// non-component arrow / function-expression VariableDeclarator (so
+// closed-over names from an outer component don't leak into the
+// helper's prop check). Three rules previously inlined an identical
+// copy — now they all reference this single source of truth.
+const isFunctionLikeVariableDeclarator = (node: EsTreeNode): boolean => {
+  if (node.type !== "VariableDeclarator") return false;
+  return node.init?.type === "ArrowFunctionExpression" || node.init?.type === "FunctionExpression";
+};
+
 export const noDerivedStateEffect: Rule = {
   create: (context: RuleContext) => ({
     CallExpression(node: EsTreeNode) {
@@ -178,13 +188,6 @@ export const noDerivedUseState: Rule = {
       return false;
     };
 
-    const isFunctionLikeVariableDeclarator = (node: EsTreeNode): boolean => {
-      if (node.type !== "VariableDeclarator") return false;
-      return (
-        node.init?.type === "ArrowFunctionExpression" || node.init?.type === "FunctionExpression"
-      );
-    };
-
     return {
       FunctionDeclaration(node: EsTreeNode) {
         if (!node.id?.name || !isUppercaseName(node.id.name)) {
@@ -470,13 +473,6 @@ export const noPropCallbackInEffect: Rule = {
       return false;
     };
 
-    const isFunctionLikeVariableDeclarator = (node: EsTreeNode): boolean => {
-      if (node.type !== "VariableDeclarator") return false;
-      return (
-        node.init?.type === "ArrowFunctionExpression" || node.init?.type === "FunctionExpression"
-      );
-    };
-
     return {
       FunctionDeclaration(node: EsTreeNode) {
         if (!node.id?.name || !isUppercaseName(node.id.name)) {
@@ -1530,13 +1526,6 @@ export const noMirrorPropEffect: Rule = {
       return new Set();
     };
 
-    const isFunctionLikeVariableDeclarator = (node: EsTreeNode): boolean => {
-      if (node.type !== "VariableDeclarator") return false;
-      return (
-        node.init?.type === "ArrowFunctionExpression" || node.init?.type === "FunctionExpression"
-      );
-    };
-
     const checkComponent = (componentBody: EsTreeNode | null | undefined): void => {
       if (!componentBody || componentBody.type !== "BlockStatement") return;
       const propNames = getCurrentPropNames();