Add Gemma3n text model support (rebased from #346)

[secret-ai-dev]

Summary

This PR adds Gemma3n text model support to mlx-swift-examples, based on the original work by @xlab in #346.

Changes

• Rebased and consolidated commits: Combined 3 commits into a single commit while preserving original authorship
• Updated file headers: Updated contributor attribution to reflect maintenance work
• Formatted code: Formatted code by swift-format
• Resolved conflicts: Fixed merge conflicts with the current main branch

Background

The original PR #346 by @xlab has been inactive for weeks. To move this valuable contribution forward, We've rebased the changes and consolidated them into a clean, mergeable state.

Credit

All implementation credit goes to @xlab for the original Gemma3n Swift port. This PR only handles the maintenance and integration work.

[xlab]

The original PR #346 is not stale. It was up for a review, which I was waiting for 2 weeks. Because CI checks are not run unless someone from owners of this repo approve, I didn't even know that it needs swift-format.

This info from #346 (comment) comment is 3 days ago, so I had to wait until weekend to run this.

If you guys really want to help bring Gemma3n here, please head to #340 and do the heavylifting for the Visual and Audio parts. 🙏

[secret-ai-dev]

Hi @xlab,

We sincerely apologize for any frustration this caused. Our team was eager to move this valuable contribution forward and didn't have permissions to collaborate directly on your PR, so we created a new one. We absolutely didn't intend to take credit for your excellent work.

Now that your PR is merged, we'll close this one immediately.