docs: update composite chunker README to reflect KeyAboveHighWatermark implementation

[morgo]

A Pull Request should be associated with an Issue.

Related: issue #479

Summary

The pkg/table/README.md stated that the composite chunker "always returns FALSE for the KeyAboveHighWatermark optimization" and that it might be implemented in the future. This is no longer accurate — issue #479 has been implemented and the composite chunker now performs real key comparisons using Datum types.

Updated the README to accurately describe the current behavior:

• KeyAboveHighWatermark works correctly for numeric, binary, and temporal primary key types
• For VARCHAR/TEXT columns with collations, Go's byte-order comparison may differ from MySQL's collation order
• Any discrepancies are caught by the checksum phase, since watermark optimizations are disabled before checksumming begins

Also verified pkg/migration/README.md — its reference to issue #479 is in a different context (deadlock reduction) and remains accurate, so no changes were needed there.

Also removes paths-ignore from CI checks. This works well with optional checks, but since our checks are required, it leaves the PR in an un-mergeable state.