fetch the expected number of rows, instead of stopping at the expecte…

[severo]

…d byte offset

fixes #28

[Copilot]

Pull request overview

This pull request modifies the CSV data frame fetching logic to fetch a specific number of rows rather than stopping at an estimated byte offset. This change aims to improve the accuracy of row fetching when byte offset estimates are imprecise, particularly in CSVs with variable row sizes.

Key changes:

• Replace byte-offset-based termination with row-count-based termination
• Add stats.valid counter to track parsed non-empty rows
• Update test expectations to reflect the new fetching behavior

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/dataframe.ts	Modified fetch logic to count rows instead of checking byte offsets; added stats.valid counter and changed loop termination condition
test/dataframe.test.ts	Updated test expectations for resolve event counts and adjusted comments to reflect that row size estimation "might" refresh (previously said "refreshes")

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