chore: replace rollup-plugin-dts with tsc for declaration emission

[kinyoklion]

Remove rollup-plugin-dts and emit type declarations directly with tsc via a new tsconfig.build.json. The build now produces lib/types/*.d.ts alongside the existing lib/cjs and lib/esm bundles; the types / exports.types entries in package.json point at lib/types/index.d.ts.

The rollup-plugin-dts was just combining all the types into a single .d.ts file for this project. It wasn't rolling up the .d.ts from its dependencies.

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Note

Medium Risk
Primarily build/publish metadata changes, but it affects the emitted/packaged TypeScript declaration layout and could break consumers if the new lib/types output differs or is incomplete.

Overview
Build output now generates .d.ts files via tsc -p tsconfig.build.json into lib/types/ (declarations-only), rather than using Rollup to bundle a single lib/index.d.ts.

package.json and exports.types are updated to point consumers at ./lib/types/index.d.ts, and rollup-plugin-dts is removed from the Rollup config and dev dependencies.

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