A5: batch v3 — aggregate + qc + outputs

[jepegit]

Epic A — Batch v3 · aggregate.py + qc.py + outputs.py. Plan §4.5.

Acceptance

• combine_summaries(cells, journal) -> pl.DataFrame — one tidy long-format frame (cell, group, sub_group), replacing the wide/multiindex join machinery.
• qc.check(cells, journal) -> pl.DataFrame — the _check_cell_* family as one function returning a tidy pass/fail frame.
• outputs.write_csv/write_excel/write_parquet — pure writers taking an explicit path from BatchPaths; never mkdir implicitly.

Blocks Epic B (collect/summary.py builds on combine_summaries). Part of #696.