chore: Document MIXED_EXPORTS rollup warning impact on CJS consumers

[untemps]

Summary

`vite build` emits this warning on the CJS bundle:

```
[MIXED_EXPORTS] Warning: Entry module "src/index.ts" is using named
(including "classifyError", "default", "isSupported", "useCommands", "useVocal")
and default exports together. Consumers of your bundle will have to use
`chunk.default` to access the default export, which may not be what you want.
Use `output.exports: "named"` to disable this warning.
```

The CJS bundle ends with:

```
exports.classifyError=O,exports.default=A,exports.isSupported=g,exports.useCommands=x,exports.useVocal=y;
```

Pure CommonJS consumers (`const Vocal = require('@untemps/react-vocal')`) need `.default` to get the component. TS/ESM consumers with `esModuleInterop` are unaffected.

Action items

• Document the CJS access pattern in the v2 release notes / README ("Migration" or "Usage with CJS").
• Decide whether to keep the mixed shape (current behavior, ergonomic for ESM) or migrate to `output.exports: "named"` + drop the default export (forcing every consumer to `{ Vocal }` — breaking).
• If keeping mixed, suppress the warning intentionally (e.g. via `output.exports: 'auto'` if Rollup still accepts the current shape).

Likely not a regression vs. 1.x, but worth confirming before the major bump.