S2: output — isatty defaults, last-query rule, assertions, EPIPE/SIGINT discipline #6

Part of #17. Design: docs/design.md §3.3, §3.4, §3.6.

Scope

• Choosing a format. -o/--output and RELIX_OUTPUT. With neither, a terminal gets table and anything else gets tsv.
• Modifiers:
  • --no-header (csv/tsv)
  • --null=STRING
  • --color=auto|always|never, honouring NO_COLOR
  • --line-buffered
• Several queries in one script:
  • table prints each, labelled.
  • A machine format prints the last query, or the one named by --query=NAME (no short form: -q is --quiet, defined in S1).
  • --all with ndjson tags each row "_query": NAME. --all with any other machine format is a usage error.
• Assertions: --fail-empty / --fail-rows exit 1.
• Pipe discipline:
  • An EPIPE-aware RowSink closes the Stream<Tuple> on a write failure and exits 141 silently.
  • A SIGINT shutdown hook closes the active stream and the session, then exits 130.
  • Output goes through a buffered stdout, flushed per result set.

Dependencies

• S1 (S1: command skeleton — script sources, exit codes, diagnostics, profiles, engine options #4), S2a (S2a: TSV and NDJSON renderers #5) for the TSV and NDJSON renderers.
• Full cancellation of in-flight JDBC/HTTP work on close is Closing a result stream cancels in-flight work (JDBC statement, HTTP fetch) relix-core#83. This slice closes the stream either way.

Acceptance

• A sink that fails after N rows: assert the stream was closed and at most N+1 rows were pulled. This is a count, not a time.
• -e 'Naturals' | head-style termination is tested.
• The isatty default is tested both ways.