docs: record #2138 in the findings index and the 17,273 baseline

SR-AUD-266's family half moves to remediated, with the finding's own wording
corrected rather than silently satisfied: 'silently misrepresented' did not
reproduce on either measurement, so this was a diagnostic defect. The index row
records all three original refusals, the two deliberate limits (the hostname
doors, left to #2359; full dual-stack, filed as #2363), and the mutation that
turned out to be equivalent code.

Author: robertvokac