chore: modernize package exports map (breaking)

[marcstraube]

Goal

Modernize package.json exports map for stricter ESM compliance.

Changes

• Review and clean up subpath exports
• Remove any legacy/compat entries
• Ensure all exports use types + import conditions
• Add sideEffects: false where applicable
• Test with all major bundlers (Vite, esbuild, webpack, Rollup)