feat: migrate build tooling from tsup to tsdown and update TypeScript

[jaredwray]

Please check if the PR fulfills these requirements

• Followed the Contributing and Code of Conduct guidelines.
• Tests for the changes have been added (for bug fixes/features) with 100% code coverage.

What kind of change does this PR introduce?

Build tooling and configuration update

Description

This PR migrates the project's build tooling from tsup to tsdown and updates TypeScript to v6. The changes include:

• Build tool migration: Replaced tsup with tsdown in the build script, removing the rimraf ./dist pre-step and --clean flag as tsdown handles this automatically
• Output file extensions: Updated entry points to use .mjs and .d.mts extensions (ES module conventions) instead of .js and .d.ts
• TypeScript upgrade: Updated TypeScript from v5.9.3 to v6.0.3
• Module resolution: Changed tsconfig.json moduleResolution from "node" to "bundler" to align with modern bundler expectations

These changes modernize the build pipeline while maintaining the same ESM output format.

Test Plan

Existing tests pass with the new build configuration. The build output remains functionally equivalent with the updated file extensions properly reflected in package.json exports.

https://claude.ai/code/session_017Khrq5EYfF7EzJcv2iT7Cp

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (0a03618) to head (d3db25c).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files            5         5           
  Lines          518       518           
  Branches       145       143    -2     
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  Hits           518       518           

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[gemini-code-assist]

Code Review

This pull request migrates the build system from tsup to tsdown, updates output file extensions to .mjs and .d.mts, and bumps the TypeScript version. Feedback highlights critical issues in the build script, specifically the removal of the directory cleaning step which could result in stale assets, and a mismatch between the default build output extensions and the entry points defined in package.json. Furthermore, the TypeScript version update to ^6.0.3 is flagged as potentially invalid.