chore: provide public API types via index.d.ts

Adds a standalone TypeScript declaration file for the public API, wires it up
via package.json "types", and ships it in the published "files". Keeping the
types in a separate .d.ts (no inline JSDoc) avoids source-file merge conflicts
while giving consumers IDE/tsc support.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: lucas-tortora