[vps] Add dnscrypt-proxy with tests

[vps] Add dnscrypt-proxy with tests

Package dnscrypt-proxy 2.1.7 (Go rewrite) via trivial-build-system
with TOML config and blocked-names.txt bundled in the store.
Shepherd daemon with respawn (no #:user — binary drops privileges
via TOML user_name).  User and group for privilege separation.
resolv.conf deployed via etc-service-type.

tests/dnscrypt.scm: Three test cases (service running, port 53
TCP, DNS resolution via getaddrinfo) following the VPS test
harness pattern.

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