feat(curl): implement -D/--dump-header (#389)

* feat(curl): implement -D/--dump-header

Write response headers to a file or stdout (-D -), matching real curl.
Fixes the loud missing-option half of vercel-labs/just-bash#388.

Co-authored-by: Cursor <cursoragent@cursor.com>
Signed-off-by: Lars Trieloff <lars@trieloff.net>

* fix(curl): address -D review gaps (redirects, -f, -v/-o, = form)

Honor --dump-header=, dump headers before -f fails, truncate the dump
file at transfer start, emit -D - with -v/-o/-w, and record intermediate
redirect hops on FetchResult for multi-block -D dumps.

---------

Signed-off-by: Lars Trieloff <lars@trieloff.net>
Co-authored-by: Cursor <cursoragent@cursor.com>