Add tests for both Node.js and browser environments

[coderabbitai]

Summary

Currently, the lack of environment-specific tests allows build regressions to slip through — as seen in #41 (partially reverted in #45).

Goal

Add tests (or CI jobs) that verify the package works correctly in both environments:

• Node.js — ensure the Node.js build is functional and not broken by changes.
• Browser — ensure the browser build is functional and not broken by changes.

Motivation

PR #41 broke the Node.js build without being caught by existing CI. Having dedicated test runs for each target environment would catch such regressions early.

Acceptance Criteria

• A test (or CI job) that exercises the Node.js build and fails if the build is broken.
• A test (or CI job) that exercises the browser build and fails if the build is broken.
• Both checks run on every PR.

References

• PR introducing the regression: Create native binding lazily #41
• PR fixing the regression: Fix build #45

/cc @tomusdrw