chore(deps): migrate tsup -> tsdown

[Pugma]

as titled

[Copilot]

Pull Request Overview

This PR migrates the build tooling from tsup to tsdown for bundling TypeScript code. The change updates the dependency, build script command, and configuration import to use the new tool.

• Updates package dependency from tsup to tsdown
• Changes build script to use tsdown command instead of tsup
• Updates configuration file import to use tsdown

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
tsdown.config.ts	Updates import statement to use tsdown instead of tsup
package.json	Replaces tsup dependency with tsdown and updates build script

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