docs(parser): document AST property order (#27038)

## Summary

- document the property order emitted by `oxc-parser`
- explain `visitorKeys` traversal order and why it is not alphabetical
- add an order-sensitive `Object.keys` regression test

## Test

- `pnpm test-node run test/visit.test.ts` (12 tests passed)

## AI assistance

Codex was used to help implement and draft this change. The contributor reviewed the resulting code and documentation and takes full responsibility for the submission.

napi/parser/README.md

@@ -31,6 +31,26 @@ The only differences between Oxc's AST and ESTree / TS-ESTree are:
 That aside, the AST should completely align with Acorn's ESTree AST or TS-ESLint's TS-ESTree.
 Any deviation would be considered a bug.
 
+#### Property order
+
+ESTree and TS-ESTree do not define an order for node properties. `oxc-parser` emits them in this
+order:
+
+1. `type`.
+2. Node-specific fields in Oxc's defined serialization order. Properties containing child nodes
+   have the same relative order as the entries in `visitorKeys[node.type]`.
+3. `start`, then `end`.
+4. `range`, when the `range` option is enabled.
+
+`visitorKeys` is an exported map from a node type to the properties containing that node's children,
+in traversal order. For example, `visitorKeys.ConditionalExpression` is
+`['test', 'consequent', 'alternate']`. This is not alphabetical order: walking `alternate` before
+`test` would not match the syntax or evaluation order of a conditional expression.
+
+Fields omitted by the selected `astType` do not change the relative order of the remaining fields.
+A generic walker which recursively inspects `Object.keys(node)` therefore visits standard ESTree
+children in the same relative order as Oxc's `Visitor` and the exported `visitorKeys`.
+
 ### AST Types
 
 [@oxc-project/types](https://npmx.dev/package/@oxc-project/types) can be used. For example:

napi/parser/test/visit.test.ts

@@ -262,3 +262,28 @@ it("visitor keys", () => {
   expect(visitorKeys.ParenthesizedExpression).toEqual(["expression"]);
   expect(visitorKeys.V8IntrinsicExpression).toEqual(["name", "arguments"]);
 });
+
+it("emits node keys in a stable order", () => {
+  const { program } = parseSync(
+    "test.ts",
+    "const fn = async <T>(arg: T): Promise<T> => await arg;",
+  );
+  const [declaration] = program.body;
+  if (declaration.type !== "VariableDeclaration") throw new Error("Expected variable declaration");
+  const [{ init }] = declaration.declarations;
+  if (init?.type !== "ArrowFunctionExpression") throw new Error("Expected arrow function");
+
+  expect(Object.keys(init)).toStrictEqual([
+    "type",
+    "expression",
+    "async",
+    "typeParameters",
+    "params",
+    "returnType",
+    "body",
+    "id",
+    "generator",
+    "start",
+    "end",
+  ]);
+});