fix(node): align ESM examples with the CommonJS runtime contract

[ubugeeei]

Problem

README documents import { Decoder, encodeEvent } from "@matesinc/fastsse-node", but the package is currently CommonJS (type: "commonjs") and exports through module.exports = require(...). A local Node 24 ESM smoke import fails with Named export not found.

Acceptance criteria

• Either add a real ESM wrapper/export map that supports named ESM imports, or update docs/types to the supported CommonJS/default import contract.
• Add a runtime smoke test for the documented import style.
• Keep CommonJS consumers working.