replace github.com/kjk/siser with github.com/kjk/common/siser

The old kjk/siser repo no longer exists. Upgrade kjk/common to latest,
adapt to ReadRecord type for Get(), and bump go to 1.25 as required
by the new kjk/common.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>