fix: stream delimited output without materializing records

[codeforester]

Fixes #409. Streams CSV and TSV rows with per-row validation and documents partial-output behavior for a later invalid record. Tests: tests/test_output.py.

[codeforester]

Review: CI is red: ruff format --check wants the row = [...] list collapsed onto one line in output.py (ruff format lib/python/base_cli/output.py). The change is otherwise correct and the new test proving the next record is only requested after the previous row is written is good.

Design note, not a blocker: this deliberately drops the prior guarantee that an invalid nested value produces no output. That is now documented in output-contracts.md, but it is a user-visible behavior change that should also get a CHANGELOG entry, and json-contracts.md still stresses all-or-nothing for JSON, so the CSV/TSV difference may be worth one sentence there.