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Improve trace shifts when one amp is masked

_continuum_subtract_median now takes the median over fibers separately
at each wavelength, using only fibers with ivar>0 at that wavelength,
and gives zero weight to wavelengths with fewer than min_fibers (20)
good fibers. Previously a fiber with good data anywhere was used at all
wavelengths, so with one amp masked about half of the fibers entered
the median as flux=0, ivar=0 over half of the spectrum. Isolated pixels
where 249 of 499 fibers were masked then got full weight with median
flux ~0, and the external sky cross-correlation aliased to shifts of
several Angstrom (e.g. Nevis 20211005/103137 r1 MEANDY +1.68 px, true
0.00 px; now +0.003 px).

Also log the desi_compute_trace_shifts command line when main() is
called with a list of arguments, and add a unit test for the masked
median.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

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