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>

buildscripts/install-verified-fixture.sh

@@ -0,0 +1,48 @@
+#!/usr/bin/env bash
+
+set -euo pipefail
+
+if [ "$#" -ne 3 ]; then
+	echo "usage: $0 SOURCE SHA256 TARGET" >&2
+	exit 2
+fi
+
+source_ref=$1
+expected=${2,,}
+target=$3
+if ! printf '%s\n' "${expected}" | grep -Eq '^[0-9a-f]{64}$'; then
+	echo "expected checksum must be a lowercase SHA-256 digest" >&2
+	exit 1
+fi
+if [ ! -d "$(dirname "${target}")" ]; then
+	echo "target directory does not exist: $(dirname "${target}")" >&2
+	exit 1
+fi
+
+tmp_file=$(mktemp "${TMPDIR:-/tmp}/silo-fixture.XXXXXX")
+trap 'rm -f "${tmp_file}"' EXIT
+case ${source_ref} in
+	https://*)
+		curl --fail --location --retry 3 --silent --show-error \
+			"${source_ref}" --output "${tmp_file}"
+		;;
+	*)
+		if [ ! -f "${source_ref}" ]; then
+			echo "fixture is not a regular file: ${source_ref}" >&2
+			exit 1
+		fi
+		cp "${source_ref}" "${tmp_file}"
+		;;
+esac
+
+if command -v sha256sum >/dev/null 2>&1; then
+	actual=$(sha256sum "${tmp_file}" | awk '{print $1}')
+else
+	actual=$(shasum -a 256 "${tmp_file}" | awk '{print $1}')
+fi
+if [ "${actual}" != "${expected}" ]; then
+	echo "fixture checksum mismatch: expected ${expected}, got ${actual}" >&2
+	exit 1
+fi
+
+install -m 0755 "${tmp_file}" "${target}"