Set row numbers on last rows

[severo]

Big refactoring, aiming at simplifying the concepts and the code, and having more predictability.

Improvements:

• stability: we refine the evaluation of row numbers only if it has changed significantly (>1%)
• stability: simplify the fetch (evaluate the byte range to parse, then fetch and parse it), avoiding previous concurrency issues
• last rows: if the end of the file was parsed, we ensure scrolling to the end of the table shows rows sticking to the end.

[Copilot]

Pull request overview

This PR refactors the CSV caching and row estimation logic to improve how row numbers are assigned, particularly for rows at the end of files. The changes separate estimation concerns from caching by introducing a new Estimator class and restructuring the CSVCache to focus purely on storage.

Key Changes:

• Introduced a new Estimator class to handle row number estimation separately from caching
• Refactored CSVCache to remove internal row tracking (firstRow) and event emission, simplifying its responsibility to pure data storage
• Updated the fetch logic to use byte-offset-based estimation rather than row-number-based positioning

Reviewed changes

Copilot reviewed 6 out of 7 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
test/dataframe.test.ts	Updated tests to reflect new estimation behavior where out-of-bound fetches no longer throw and estimated row numbers are returned even for uncached rows
test/cache.test.ts	Restructured tests to cover the new RowsCache and Estimator classes, removing tests for the old row-tracking approach
src/helpers.ts	Added checkInteger function for validating integer values without the non-negative constraint
src/dataframe.ts	Integrated Estimator class, refactored fetch logic to use byte offset estimation, and updated event handling to refresh estimates explicitly
src/cache.ts	Major refactoring: introduced RowsCache and Estimator classes, removed firstRow tracking from CSVRange, simplified CSVCache to focus on storage operations
package.json	Updated baseline-browser-mapping dependency from 2.8.12 to 2.9.5
package-lock.json	Lockfile update for the dependency version change

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