test: add package entrypoint smoke tests for every export

[collinsezedike]

Summary

Adds smoke tests that import every published package entry point the way a consumer does, so a broken exports map, a missing build artifact, an accidental Node-only import, or an unresolvable declaration file fails CI before release.

Checks

Fixtures live in test/smoke/ and run via pnpm test:exports after pnpm build:

• ESM: import every exports subpath through its import condition and assert it exposes exports.
• CommonJS: require every exports subpath through its require condition and assert it exposes exports.
• Node-free: walk each entry point's authored source graph and fail on any node:* or bare Node builtin, keeping the browser-safe entry points browser-safe.
• Types: assert every types target exists and type-checks when imported by its package specifier with tsc --noEmit.

The entry point list is derived from package.json exports, so new subpaths are covered automatically once they ship in the build.

CI

A dedicated entrypoints job builds the package and runs the suite across Node 20 and 22.

Notes

The Node-free check scans authored source rather than bundled output. A third-party dependency can select a Node crypto shim through export conditions, which would flag the bundle even though the package's own code is browser-safe. Scanning source targets the intended guard: code authored in this package never importing a Node-only module.

The CI workflow is covered by CODEOWNERS, so the workflow change requires maintainer review.

Closes #205

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[truthixify]

Merged. The entrypoint checks cover ESM, CJS, declarations, and browser-safe imports cleanly. Thanks @collinsezedike.