Migrate from tsup to tsdown

[Silent-Watcher]

Description:
We currently use tsup for bundling, but it is no longer actively maintained. The recommended replacement is tsdown, which offers:

• Actively maintained and modern replacement for tsup
• TypeScript-first design with better defaults
• Improved performance and DX
• Easier migration (drop-in replacement in most cases)

Tasks:

• Remove tsup from dependencies
• Install and configure tsdown
• Update build scripts (package.json → "build": "tsdown")
• Verify build outputs are consistent with previous setup
• Update docs/README to reflect tsdown usage

Why:

• Future-proof build process
• Reduce dependency on unmaintained packages
• Align with industry trend (tsdown becoming the successor to tsup)