A8: batch v3 — process-pool executor + handover

[jepegit]

Epic A — Batch v3 · Phase 3 — process-pool executor + collectors handover. Plan §6 Phase 3.

Acceptance

• executor="processes" reuses load_cell (deletes the 320-line parallel_update clone); Windows-pickle-safe (results as frames + paths, not live objects).
• Collectors interface handover point (BatchResult/tidy summaries) for Epic B.
• Progress via callback (facade passes tqdm); engine layer never imports tqdm/prints.

Note: deletion of dead stubs (ImpedanceExperiment, stub engines, batch_reporters.py, OriginLabExporter) is breaking-adjacent and moves to E4. Part of #696.