S2a: TSV and NDJSON renderers #5

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

Scope

Two streaming renderers in the command's render/ package:

• OutputFormat.TSV: IANA TSV with a header row. A tab, newline, carriage return or backslash inside a value is escaped as \t, \n, \r or \\, so a row is always one line, as cut, awk and sort expect.
• OutputFormat.NDJSON: one JSON object per line, in the same value encoding the JSON format uses.

Both take a header option and a NULL spelling, which the command exposes as --no-header and --null.

Dependencies

• S0b (S0b: console support as internal packages of the command #3).

Acceptance

• Golden tests per format, covering every value type, escaping, NULL and nested values.
• A streaming test shows the first row is written before the input ends.