A7: batch v3 — utils.batch shims + docs migration

[jepegit]

Epic A — Batch v3 · Phase 2 — shims + docs migration. Plan §6 Phase 2.

Acceptance

• cellpy.utils.batch re-implemented as shims over cellpy.batch (signatures preserved; exotic kwargs → LoadPolicy fields or a helpful DeprecationWarning naming the replacement).
• init/init2/naked/from_journal2/process_batch/iterate_batches → deprecated thin wrappers (iterate_batches/process_batch become docs recipes, decided 2026-07-26).
• batch_tools modules gain import-time DeprecationWarning.
• Published old→new symbol map; repo docs/tutorials/notebooks moved to the new API.

Non-breaking (warn-only). Removal of batch_tools itself is E4. Part of #696.