B3: collectors — class re-based, no constructor I/O

[jepegit]

Epic B — Collectors · Phase 2 — class re-based. Plan §4 Phase 2.

Acceptance

• collector.py BatchCollector = options + collect + plot + save; constructors construct (no full-batch load / parse_units I/O in __init__; unit check happens during collect(), result in meta).
• BatchSummaryCollector/BatchICACollector/BatchCyclesCollector → one-line shims mapping elevated kwargs → options fields (unknown kwargs raise with the valid field list).
• standard_gravimetric_collector re-implemented as the declarative transforms recipe (§3.4); old callable delegates (kept as a shim on the 2.1 cadence).

Part of #696.