feat: add progress bar. ref #44.

[tompollard]

As highlighted in #44, it would be helpful to provide the user with feedback on progress during creation of Croissant metadata, particularly for large datasets.

This pull request:

• Moves creator parsing and CSV warning logic before the progress context
• Adds per-file progress bar during metadata generation
• Keeps the spinner for the save/validate phase.

Testing:

• Run croissant-baker --input <dataset_dir> on a dataset with multiple files and confirm per-file progress bar is displayed
• Verify the progress bar shows file count, percentage, and current file name
• Verify save/validate spinner still works as before

[rafiattrach]

Nice work! Thanks for taking care of it @tompollard ! I tested this locally on both the bundled MIMIC-IV Demo (32 files) and full MIMIC-IV (~10 GB, 31 compressed CSVs). Progress bar works well, no performance regression, and output is identical.

One thing I noticed: the progress bar total includes discovered files that have no handler (e.g. index.html, README.txt). On the demo dataset it shows 38/38 but the summary says "Files: 32". Since we don't know which files have handlers until we try each one, the simplest fix would be changing the label from "Processing files..." to "Scanning files..." — that way the count covering all discovered files feels natural.

[tompollard]

Thanks Rafi!

Since we don't know which files have handlers until we try each one, the simplest fix would be changing the label from "Processing files..." to "Scanning files..."

Good catch, I updated the text as suggested.