A6: batch v3 — facade + golden parity

[jepegit]

Epic A — Batch v3 · facade.py + __init__.py. Plan §4.6.

Acceptance

• Batch facade: pages, summaries (cached), update, plot (delegates), report, save, mark_as_bad, drop, cells (+ data alias).
• One public constructor: load(name, project, *, journal=None, db=None, frame=None, policy=None) + from_journal().
• Golden parity vs the old implementation on shared columns (mapped through header renames). A1's golden runs against both.

Part of #696.