`peek rows f.csv --fields id,price --json` projected correctly; the same
command with --text printed every column and said nothing. A flag that
works in one format and is silently dropped in another is worse than one
that does not exist, and --fields is a CommonOption, so every
TableRenderable payload in the fleet had it.

Names match the COLUMN HEADINGS in text mode, case-insensitively,
because that is what somebody reading a table can see and therefore what
they type. JSON keeps matching the underlying keys: a heading reading
SHARE may be a `percent` field, and there is no mapping between them to
guess at. Each format matches what it shows.

A name that hits nothing warns and keeps the whole table. Emptying it
would read as "no data" rather than "wrong flag".

Flexible column indices are renumbered against the surviving set, or the
wrong column absorbs the slack. 5 tests.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01SNXBmZ6Ad8bc8GhCD1exWx