Pinned upgrade artifact routing

Chose target-derived artifact routing over persisted-channel routing because one-off exact pins must not alter tracking state or query unrelated latest metadata. Exact pins bypass latest discovery and retain their resolved source; nightly targets always use standalone/GHCR and migrate non-curl installs, while stable package-manager targets use registry validation and stable Homebrew pins fail before network or subprocess work. Changelog behavior follows the target; setup/result channel preserves the stored tracking channel.