test: pin the external fixtures and run the suites against the silo binary

The test and verification scripts invoked ./minio and pulled their tooling from
upstream infrastructure with no integrity check. Every `curl | tar` of a client
or an old server binary was an unverified execution path in a script that
regularly runs as a privileged user, and several fetched a floating "latest".

Two installers replace all of it:

- install-mcli.sh resolves a pinned pgsty/mc release, downloads the archive and
  its checksum manifest, requires exactly one valid manifest entry for the
  asset, verifies it, and installs. MCLI_BIN with a mandatory MCLI_SHA256 lets
  an offline or air-gapped run supply its own binary, still checksum-checked.
- install-verified-fixture.sh takes source, expected SHA-256 and target, and
  refuses anything that does not match. Sources may be a URL or a local file.

Every script that previously downloaded mc now calls install-mcli.sh. The three
places that genuinely need an upstream artifact - the old MinIO server binary
for the LDAP IAM upgrade-import test, the 2021 mc for the three-site
replication test, and the functional-tests.sh fixture - go through
install-verified-fixture.sh with the digest recorded inline. Those dl.min.io
URLs remain on purpose: they are historical upstream artifacts needed to prove
upgrade compatibility, and they are now pinned and verified rather than
trusted.

The scripts otherwise switch to ./silo, silo.service, the silo container and
compose service names, and SILO_CONFIG_DIR. run-multi-site-minio-idp.sh is
renamed to run-multi-site-silo-idp.sh with the Makefile target following.
buildscripts/minio-upgrade.sh keeps its name and its `minio server` argv - it
exists to test the MinIO-to-Silo upgrade, so the old side must stay old - but
it is now pinned to an image digest rather than a tag, and its `docker system
prune` and `docker volume prune` calls are removed. Those ran unfiltered
against the developer's whole Docker installation; the resiliency tests had the
same problem and lose their prune and `docker ps -q` sweeps too.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/federation/lookup/Corefile.example

@@ -1,5 +1,5 @@
 . {
-  etcd churchofminio.com {
+  etcd silo.example {
       endpoint http://localhost:2379 http://localhost:4001
       upstream /etc/resolv.conf
   }
@@ -8,4 +8,4 @@
   cache 160 mydomain.com
   loadbalance
   forward . /etc/resolv.conf
-}
+}