Add multi-PDS software support with per-software version tracking
Servers running alternative PDS implementations (millipds, cirrus, cocoon,
etc.) are now detected by matching their version against known GitHub tags
and compared against their own software's latest release for the trust
score. Link-only software without version tracking gets benefit of the
doubt. Software name shows as a clickable link in the directory. JSON API
includes per-software latest versions alongside the existing field.