fix: emit package type declarations

[mawxcodehub]

Summary

• enable tsup declaration generation so the package emits the advertised dist/index.d.ts
• keeps the existing ESM output, exports map, and package files unchanged

Validation

• npm run typecheck
• NODE_OPTIONS=--max-old-space-size=4096 npm run build
• npm pack --dry-run --json --ignore-scripts includes dist/index.d.ts
• local ESM consumer install from the packed tarball passes npx tsc --noEmit --pretty false

Fixes #6