chunk_text/hard_split/split_paragraphs + 2400/600-char constants move out of
documents_ingest.chunk_embed into kb_mail.chunking so the upcoming
jobs/mail-body-ingest shares the exact same chunker instead of a copy-pasted
drift (the phase-4 lesson for retrieval.py -> packages/kb-retrieval, applied
again). chunk_embed.py re-exports unchanged; zero behavior change, tests moved
1:1 alongside the code (docs/kb/modules/05-faza-mailowa-plan.md, §3).
