fix(installer): harden greenfield detector contracts

tools/verified-installer-fetch.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env bash
+# Fetch, authenticate, and execute the exact downloaded installer body.
+set -euo pipefail
+url="${1:?usage: verified-installer-fetch.sh <url> <sha256> [-- installer-args...]}"
+expected="${2:?usage: verified-installer-fetch.sh <url> <sha256> [-- installer-args...]}"
+shift 2
+[[ "${1:-}" != -- ]] || shift
+[[ "$expected" =~ ^[0-9a-f]{64}$ ]] || { echo 'installer expected SHA-256 must be 64 lowercase hex characters' >&2; exit 2; }
+tmp="$(mktemp "${TMPDIR:-/tmp}/mosaic-installer-body.XXXXXX")"
+trap 'rm -f "$tmp"' EXIT
+chmod 0600 "$tmp"
+curl -fsSL "$url" -o "$tmp"
+[[ -s "$tmp" ]] || { echo 'installer fetch returned an empty HTTP-success body' >&2; exit 1; }
+actual="$(sha256sum "$tmp" | awk '{print $1}')"
+[[ "$actual" == "$expected" ]] || { echo "installer SHA-256 mismatch (got=$actual expected=$expected)" >&2; exit 1; }
+status=0
+bash "$tmp" "$@" || status=$?
+rm -f "$tmp"
+trap - EXIT
+exit "$status"